monograph series

4

Narcotic

Antagonists:

The Search for

Long-Acting

Preparations
The NIDA Research Monograph series is prepared by the

Research Division of the National Institute on Drug Abuse. Its

primary objective is to provide critical reviews of research problem

areas and techniques, the content of state-of-the-art conferences,

integrative research reviews and significant original research. Its

dual publication emphasis is rapid and targeted dissemination to

the scientific and professional community.

EDITORIAL ADVISORY BOARD

Avram Goldstein, M.D. Addiction Research Foundation

Palo Alto, California

Jerome Jaffee, M.D. College of Physicians and Surgeons

Columbia University, New York

Reese T. Jones, M.D. Langley Porter Neuropsychiatric

Institute

University of California

San Francisco, California

William McGlothlin, Ph.D. Departrnent of Psychology, UCLA

Los Angeles, California

Jack Mendelson, M.D. Alcohol and Drug Abuse Research

Center

Harvard Medical School

McLean Hospital

Belmont, Massachusetts

Helen Nowlis, Ph.D. Office of Drug Education, DHEW

Washington, D.C.

Lee Robins, Ph.D. Washington University School of

Medicine

St. Louis, Missouri

NIDA RESEARCH MONOGRAPH series

Robert DuPont, M.D. DIRECTOR, NIDA

William Pollin, M.D. DIRECTOR, NIDA RESEARCH DIVISION

Robert C. Petersen, Ph.D. EDITOR-IN-CHIEF

Eunice L. Corfman, M.A. MANAGING EDITOR

Rockwall Building 11400 RockviIle Pike RockviIle, Maryland, 20852
monograph series

Narcotic

Antagonists:

The Search for

Long-Acting

Preparations

Editor

Robert Willette, Ph.D.

Division of Research

National Institute on Drug Abuse

January 1976

THE NATIONAL INSTITUTE ON DRUG ABUSE

11400 Rockville Pike

Rockville, Maryland 20852

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 - Price $1.10

Stock Number 017-024-00488-0
DHEW Publication No. (ADM) 76-296

Printed 1975

Library of Congress catalog number : 75-29949

ii
Molded beads of 25/75 lactic/glycolic copolymer

50wt% naltrexone pamoate mixture. (mm)

Dynatech R/D. (See first article).

Subcutaneous scapular implant site of Dl-6

five weeks after implantation in rabbit.

Note the nodular mass (arrow).

iii
Subcutaneous scapular implant site of D2-7

five weeks after implantation.

Note rod-shaped residues (arrow).

Subcutaneous scapular implant site of D2-8

five weeks after implantation.

Note rod-shaped residues (arrows).

i v
CONTENTS

INTRODUCTION

ABSTRACTS 7

DEVELOPMENT OF POLYLACTIC/GLYCOLIC ACID

DELIVERY SYSTEMS FOR USE IN TREATMENT OF

NARCOTIC ADDICTION

A.D. Schwope, D.L. Wise, J.F. Howes 13

DEVELOPMENT OF INJECTABLE MICROCAPSULES

FOR USE IN THE TREATMENT OF NARCOTIC

ADDICTION

C. Thies 19

LONG-ACTING NARCOTIC ANTAGONIST COMPLEXES

A.P. Gray, W.J. Guardina 21

SUSTAINED RELEASE OF NALTREXONE

FROM GLYCERIDE IMPLANTS

M.F. Sullivan, D.R. Kalkwarf 27

USE OF SYNTHETIC POLYPEPTIDES IN THE

PREPARATION OF BIODEGRADABLE DELIVERY

VEHICLES FOR NARCOTIC ANTAGONISTS

K.R. Sidman, D.L. Arnold, W.D. Steber,

L. Nelsen, F.E. Granchelli, P. Strong,

S.G. Sheth 33

DEVELOPMENT OF CHRONOMERSTM

FOR NARCOTIC ANTAGONISTS

R.C. Capozza, E.E. Schmitt, L.R. Sendelbeck 39

TESTING OF DRUG DELIVERY SYSTEMS

FOR USE IN TREATMENT

OF NARCOTIC ADDICTION

R.H. Reuning, L. Malspeis, S. Frank,

R.E. Notari 43

v

INTRODUCTION

The use of narcotic antagonists in the treatment

of opiate addiction is based on the concept of a

pharmaceutical agent capable of blocking the reinforcing properties of a dose of opiate taken

during an addicts rehabilitation. The rationale

for use is that the antagonist blocks the opiate

"high" and makes it pleasureless, thus removing

the addict's incentive for continued use. Earlier

successful therapy with cyclazocine and naloxone

prompted the full-scale development of new and

superior antagonists. Presently naltrexone is

the drug under the most intensive clinical

evaluation and appears to be a promising antagonist candidate.

It was felt from the outset that a most desirable

component of antagonist therapy would be a longacting drug, so that the need for an addict to

decide to take his medication would be minimized.

Naltrexone in oral doses of 70 mg. will provide

adequate blocking protection for at least 48

hours, or perhaps 72 hours in certain individuals.

This is not felt to be a long enough interval between dosages to aid the addict in becoming dissociated from his drug-taking behavior.

It was recognized very early that in order to

achieve the desired one week, one month or longer

duration between dosages, it would be necessary

to develop a long-acting drug delivery system or

a sustained-release preparation of an acceptable

but short-acting antagonist. A "drug-delivery

system" is the unwieldy but currently favored

expression describing any pharmaceutical preparation capable of providing a sustained or longacting antagonistic effect. This effect may be

achieved mechanically (e.g., by implanted discs

with timed release capacity) or chemically (e.g.,

1
microcapsules, tubes, solid balls, gelatinous

masses injected intramuscularly). Distinct

from the problem not considered here, of finding an optimum antagonist, is the problem of

inventing suitable carriers for the antagonist,

carriers that will deliver the antagonist, releasing it uniformly bit by bit over a period

of time.

system."

Hence the barbarism, a "drug delivery

Efforts to achieve satisfactory drug delivery

systems were launched in the early 1970's by

the City of New York Public Health Department

and by the Division of Narcotic Addiction and

Drug Abuse, now the National Institute on Drug

Abuse (NIDA).

During this early period, the pioneering efforts of

Dr. Seymour Yolles, University of Delaware,

demonstrated for the first time that a sustainedrelease of an antagonist could be obtained from

a biodegradable polymer, i.e., polylactic acid.

This success generated expanded and intensified

efforts, a summary of which is the topic of

this monograph.

At the present time, the program supported by

NIDA includes six contracts that are concerned

with the development of new delivery systems

and three contracts that have the responsibility

of evaluating them for potential clinical trials.

Delivery Systems Development

Borrowing on its experience in the antifertility

area supported by the Center for Population Research, at the National Institute for Child

Health and Human Development, Dynatech Corporation has had the major goal of developing biodegradable devices from polyactic and polyglycolic acids. This effort is now centered on

refining copolymer preparations with naltrexone

that are implantable and have either a one or six

month duration.

2
Another project receiving early support has been

that of Dr. curt Thies, Washington University,

an effort to develop microencapsulated dosage

forms. Working primarily with the polylactic and

glycolic acids, Dr. Thies achieved early success

of multiple-week release with the antagonist

cyclazocine. As naltrexone became the drug of

choice, it was substituted for cyclazocine. The

highly experimental nature of these systems was

revealed when it was quickly found that the physicochemical properties of naltrexone posed new

problems. Among these was increased water

solubility. Notable progress has now been made

in improving capsule quality to achieve longer

release times.

One of the approaches supported originally by

New York City that has yielded very promising

sustained-release candidates has been directed

by Dr. Alan Gray, Illinois Institute of Technology Research. He has found that zinc and

aluminum tannate complexes of naltrexone, injected in 2% aluminum monostearate in sesame oil

gives sustained release from three to four

weeks. Advanced studies are now underway to

evaluate its safety in order to initiate

clinical trials.

Another early project that is now approaching

final evaluation for clinical trials is based

on implantable pellets of naltrexone in polyglycerides. This work has been conducted by

Drs. Maurice Sullivan and Donald Kalkwarf, of

Battelle Pacific Northwest Laboratories. The

natural polyglycerides used have shown excellent biocompatibility, that is, negligible

inflammation on insertion and little tissue

reaction overall.

A newer contract with Arthur D. Little Inc.

is concentrating on the develooment of new biodegradable polymers based on polypeptides.

Tubes of polyleucine-polyglutamate filled with

naltrexone appear to offer a promising new

lead.

3
One of the more unique approaches is that being pursued by Alza Research. In contrast to

the other polymer based release systems, which

diffuse drugs, the Alza chronomer erodes at a

rate equal to drug release. Interesting induction periods and solubility effects are currently being studied to perfect a desirable

release rate.

Evaluation

The evaluation of leading candidate devices

developed by these contractors is initiated

by a group headed by Dr. Richard Reuning at

Ohio State University. The first series of

tests consists of following the blocking of

morphine-analgesia in mice by the test device

to confirm any original test results. Next,

the release of radiolabeled drug from the

device is followed in rats and finally in

monkeys. In order to carry out this type of

evaluation, this group has conducted extensive

pharmacokinetic studies on naltrexone. This

information permits a determination of the

actual rate of release of drug from the device

under testing.

Candidate devices that pass the first two tests

at Ohio State are then submitted to Industrial

Bio-Test, Inc., for evaluation of their tissue

compatability, and to Parke, Davis and Company

for a rigorous final pharmacological evaluation.

For the toxicity studies, mice, rats, rabbits

and monkeys receive various loadings of the test

device and tissue reactions are checked, usually

at 7 and 28 days. The pharmacological studies

are carried out in a group of monkeys trained to

lever-press for injections of morphine. A

successful device will block this behavior as

long as drug is being released. Simultaneously,

blood specimens are collected for the Ohio State

group to analyze for drug levels.

4
As a result of these efforts to date, two or

possibly three candidates are being readied for

clinical trials. The final preclinical work

necessary includes sterility checks, more detailed toxicity studies, and quality control

standards. It is our hope to be able to

evaluate at least one of these devices in man

within a year.

Robert E. Willette, Ph.D.

Division of Research

National Institute on Drug Abuse

September, 1975

5

ABSTRACT

RESEARCH MONOGRAPH #4, NATIONAL INSTITUTE ON

DRUG ABUSE. Narcotic Antagonists: The Search

for Long-Acting Preparations.

DEVELOPMENT OF POLYLACTIC/GLYCOLIC ACID

DELIVERY SYSTEMS FOR USE IN TREATMENT OF

NARCOTIC ADDICTION

in vivo efficacy has been-measured by direct

challenge with morphine (Dewey-Harris mouse

Schwope, A.D., D.L. Wise, J.F. Howes

Implantable polylactic/glycolic acid matrix

systems have successfully provided the sustained release of naltrexone to mice for

periods of up to 200 days. In vitro and in

vivo release rates have been determined by

measuring chemical concentrations in pH 7

buffer solution and urine, respectively, and

tail-flick test). Dosage forms of small implantable cylinders, 1/16" diameter, (25 mg/rod,

one rod/mouse) containing 33% by weight naltrexone pamoate in 90 L(+)/10 polylactic/

glycolic acid have sustained the delivery of

chemical for 200 days. Delivery of chemical

from dosage forms of 1/16" diameter spherical

beads (3 mg/bead, 3 beads/mouse) containing

33% by weight naltrexone base in 90 L(+)/10

polylactic/glycolic acid was sustained for 60

days. Earlier a similar bead type dosage form

of 75 L(+)/25 polylactic/glycolic acid containing 50% by weight naltrexone base and coated

with the pure polymer provided controlled

release for 25 days.

which incorporate the use of pharmacologically

suitable catalysts and yield products reproducibly have been delineated. Techniques for

sterilization of the final implant have been

screened.

ABSTRACT

RESEARCH MONOGRAPH #4, NATIONAL INSTITUTE ON

DRUG ABUSE. Narcotic Antagonists: The Search

for Long-Acting Preparations.

DEVELOPMENT OF INJECTABLE MICROCAPSULES

FOR USE IN THE TREATMENT OF NARCOTIC

ADDICTION

Thies, C.

Injectible microcapsules containing narcotic

antagonists have been prepared with dl-poly

(lactic acid) as the coating material. The

encapsulation technology has developed to

the point that high yields of less than

180 μ capsules can be prepared routinely.

Such capsules with an initial payload of

50 wt. % naltrexone pwnoate provide 60-90%

atagonism to the action of morphine 28 days

after injection into mice as a peanut oil/

aluminum monostearate suspension at a dose

level of 40 miligrams naltrexone pamoate/ kg.

mouse.

ABSTRACT

RESEARCH MONOGRAPH #4, NATIONAL INSTITUTE ON

DRUG ABUSE. Narcotic Antagonists: The Search

for Long-Acting Preparations.

LONG-ACTING NARCOTIC ANTAWNIST COMPLEXES

Gray, A.P., W. J. Guardina

We evaluated the ability of close to 100

organic acids to form water-soluble salts

with methadone, cyclazocine, naloxone,

naltrexone and, more recently, diprenorphine.

About half the acids yielded insoluble salts.

Polybasic acids affording insoluble salts

were evaluated for their ability to form

drug:acid:metal complexes with the polyvalent

metal ions, Zn++, Al+++, Mg++ and Ca++

.

Optimum conditions for forming complexes have

been developed and the consistency of their

composition has been established.

Salts were analyzed spectrophotometrically

for drug content, and complexes were analyzed

for drug and metal content. The in vitro

degree of dissociation at equilibrium

measured for the preparations suspended in

a simulated physiological buffer, pH 7.3.

Preparations of the narcotic antagonist

drugs showing relatively low degrees of

dissociation in vitro, since it-early

appeared that a high degree of dissociation

contraindicated a prolonged duration of

pharmacologica1 action, were evaluated in

mice after intramuscular administration at

several dose 1evels by the mouse tail-flick

test for the potency and duration of their

morphine antagonist activity.

Our most promising preparations to date,

showing the most prolonged durations of

action without evidence of gross toxicity,

are naltrexone zinc tannate and naltrexone

aluminum tannate. These are undergoing

detailed evaluation as potential clinical

candidates. Thus far, the most useful of

several dosage forms studied is a suspension in an aluminum monostearate gel.

The abstracts are provided as

summary and may be extracted

for reference and filing convenience.

7

ABSTRACT

RESEARCH MONOGRAPH #4, NATIONAL INSTITUTE ON

DRUG ABUSE. Narcotic Antagonist's: The Search

for Long-Acting Preparations.

SUSTAINED RELEASE OF NALTREXONE

FROM GLYCERIDE IMPLANTS

Sullivan, M. F., D. R. Kalkwarf

Solid dispersions of naltrexone in natural

glycerides were used to form injectable implants which continuously release narcotic

antagonists in vivo. The dispersions were

formed and tested either as small cylindriCal pellets, e.g. lx3.0 mm in size, or

as particles with diameters in size ranges

between 125-250 μ, that are suspended in

an aqueous methyl cellulose solution.

Both types of implants delivered naltrexone to mice at rates that were effective

in blocking the antiociceptive action of

morphine for at least one month. The rate

of naltrexone release was controlled by

altering its concentration in the dispersion and by varying the glyceride composition. Degradation and absorption of the

implants were found to depend on their

composition, dimensions and location in

the body. No appreciable tissue incompatibility was seen in mice, rats, rabbits,

monkeys and swine, even when long-lasting

preparations were removed a year after

treatment.

-

ABSTRACT

RESEARCH MONOGRAPH #4, NATIONAL INSTITUTE ON

DRUG ABUSE. Narcotic Antagonists: The Search

for Long-Acting Preparations.

USE OF SYNTHETIC POLYPEPTIDES IN THE

PREPARATION OF BIODEGRADABLE DELIVERY

VEHICLES FOR NARCOTIC ANTAGONISTS

Sidman, K.R., D.L. Arnold, W.D. Steber,

L. Nelsen, F.E. Granchelli, P. Strong,

S.G. Sheth

Synthetic polypeptides consisting of copolymers of glutamic acid and leucine have been

shown to be useful materials for the fabrication of practical, biodegradable delivery

vehicles for narcotic antagonists.

Model delivery vehicles in film form were prepared from copolymers containing 10 mole percent to 40 mole percent glutamic acid, and

loaded with 10% to 40% naltrexone by weight.

The naltrexone was found to be released by

diffusion, exhibiting diffusion coefficients

that varied as a function of the glutamic acid

content and the initial naltrexone loading.

A wide range in diffusion coefficients were

achieved (0.31 x 10-7 cm

2

/hr to 120 x 10-7 cm

2

/

hr), leading to release rates within practical

ranges of interest for meeting the program

goals.

We have demonstrated that the polypeptides

can be fabricated into dosage forms that are

amenable to administration by trochar. For

example, rods 0.4 mm to 0.8 mm in diameter

containing as much as 40% naltrexone by weight

were extruded using a simple compression mold

and die arrangement. An in vitro evaluation

of the rods showed that antagonist is released

by diffusion at a continuously decreasing

rate, a behavior similar to that observed with

the film devices that were, nonetheless,

capable of blocking an AD80 challenge of

morphine sulfate in mice for more than 30

days.

One of the most promising delivery vehicles

that we have developed to date consists of a

polypeptide tube filled with a naltrexone/

polypeptide core. Preliminary experiments

have shown that these devices may be capable

of administering high, constant rates of release for prolonged periods of time. Additional work, however, is required to develop

techniques for the preparation of reproducible

delivery vehicles.

I

9

ABSTRACT

RESEARCH MONOGRAPH #4, NATIONAL INSTITUTE ON

DRUG ABUSE. Narcotic Antagonists: The Search

for Long-Acting Preparations.

DEVELOPMENT OF CHRONOMERSTM FOR

NARCOTIC ANTAGONISTS

Capozza, R.C., E.E. Schmitt, L.R. Sendelbeck

The object of this program is to prepare a

bioerodable naltrexone deLivery system which

can be implanted subcutaneously in humans and

which can relieve the narcotic antagonist

over 1-6 months at relatively constant and

sufficient rates to block the euphoric effect

of morphine based drugs. The system is composed of naltrexone uniformly dispersed in a

solid hydropholic CHRONOMERTM matrix which

undergoes predictable surface erosion when

exposed to an aqueous medium. Kinetic studies

in vitro have been carried out during the

course of the program to determine the best

composition for the system.

Toxilogical studies conducted at ALZA during

the past 2 years have not revealed limiting

adverse effects of either the CHRONOMERTM

materials or their hydrolysis products. The

tail-flick test procedure was used to measure

the effectiveness of naltrexone to antagonize

the analgesis of morphine in rats. Naltrexone

infused intravenously at doses of 4 and 16

ug/kg/hr resulted in, after 6 hours, 54 and

89% antagonism, respectively, against a

63.5% effective dose of morphine.

Preliminary Sterilization studies; showed that

no adverse effects to CHRONOMERTM/naltrexone

systems occurred after exposure to 2.5 or 5.0

mrads of 60Co irradiation.

ABSTRACT

RESEARCH MONOGRAPH #4, NATIONAL INSTITUTE ON

DRUG ABUSE. Narcotic Antagonists: The Search

for Long-Acting Preparations.

TESTING OF DRUG DELIVERY SYSTEMS FOR USE IN

TREATMENT OF NARCOTIC ADDICTION

Reuning, R.H., L. Malspeis, S. Frank,

R.E. Notari

The evaluation of the drug release characteristic of four naltrexone delivery systems has

been carried out together with the development

of analytical techniques and an investigation

of the metabolic profile of naltrexone. Pharmacologic evaluation of the four delivery systems

in the mouse indicated significant analgesic

antagonism for a period of from 16-22 days.

Further evaluation of one of these systems by

measurement of the rate of excretion of radio-

-activity after administration of radiolabelled

naltrexone in the delivery system confirmed that

significant release occurs for a time period of

about 15 days. Electron capture gas-liquid

chromatographic assays for naltrexone and naloxone in plasma or urine have been developed

that yield linear calibration curves and are

sensitive to one ng/ml. Studies on naltrexone

disposition indicate that (a) binding to plasma

proteins in several species varies from 20-26%,

(b) distribution of drug from blood is extremely

rapid and extensive, (c) -naltrexol is a major

metabolite of naltrexone in man, monkey and

guinea pig among six species studied, whereas

a-naltrexol is a minor metabolite in the monkey

and guinea pig only, and (d) metabolic reduction

of naltrexone occurs in the 100,000 x g supernatant of guinea pig liver. Pharmacokinetic

studies of naltrexone in the dog and monkey

indicate that the drug is rapidly distributed

and eliminated, has a very large apparent volume

of distribution and a total body clearance

greater than the rate of liver blood flow.

11

DEVELOPMENT OF

POLYLACTIC/GlYCOLIC ACID

DELIVERY SYSTEMS

FOR USE IN TREATMENT OF

NARCOTIC ADDICTION

Arthur D. Schwope

Donald L. Wise, Ph. D.

Dynatech R/D Company

John F. Howes, Ph. D.

Sharps Associates

INTRODUCTION

A problem universal to chemotherapeutic This is achieved by injecting the agent

programs is the administration and in a carrier solution (e.g., oil and

maintenance of drug(s) at safe and ef- saline) and by encapsulating or surfective levels for extended periods. rounding the chemical with an absorbMuch technical activity has been able, release-regulating material.

devoted to devising improved and different means for regulating drug delivery. In this study, polymers of lactide and

glycolide were evaluated as the releaseOne important category of dosage forms regulating materials. Polylacticl

under investigation is implantable, glycolic acids are tissue compatible

tissue absorbable preparations which and hydrolyze to the metabolites, lactic

release chemical (e.g., medicaments, and glycolic acids. The rate of

fertilizers, and pesticides) at a con- hydrolysis is controllable and dependent

stant rate to biological systems. There on the, lactic/glycolic ratio. Physical

are two fundamentally different ap- mixtures, matrices, of narcotic antagoproaches to achieve that end. One nist - naltrexone - and polymer were

relies on chemical alteration of the prepared, formed and evaluated in vitro

agent so that adsorption by the system and in vivo as long term, constant

is delayed. In the second method, the delivery, dosage systems.

active agent remains chemically unchanged, but its availability to the

body is restricted by physical means.

This paper describes the procedures

which have been used to prepare and

13
evaluate drug/polymer matrix systems

which will provide the sustained

release of naltrexone for one month and

and for six months. Results of the

evaluations of selected systems are

presented.

METHODS AND MATERIALS

Sample preparation

L(+)-lactide and glycolide, synthesized

from the respective acids, were polymerized into polylactic acid and polylactic/glycolic acid copolymers using

triethylaluminum and suitable catalysts. Polymers ranging from 75 wt %

L(+)lactide/25% G to 100% L(+) have

been prepared. Molecular weights of

the polymers determined by gel permeation chromatography, membrane osmometry

and light scattering ranged from 40,000

- 200,000. Intrinsic viscosities

measured at 37°C in tetrahydrofuran

ranged from 0.4 to 1.0.

Naltrexones base and pamoate (supplied

by NIDA) were used. The base was

tritium labeled at the 15 and 16 positions.

Chemical and polymer were blended into

an intimate, uniform mixture using the

solvents tetrahydrofuran and methylene

chloride. The solution was cast and

the solvent removed by evacuation

(<1 mm Hg) at 50°C for at least 24

hours. After drying, the chemical/

polymer matrix was formed into implantable shapes. Beads 1/16" diameter were

prepared by transfer molding and 1/16"

diameter rods by extrusion. A thin,

polymer coating was added to selected

samples by dipping the beads or rods

into a 10 wt % polymer solution. Upon

removal from the dip-coat bath, the

sample was dried under vacuum at 50°C

for 48 hours. The resultant coating

was estimated to be 0.001 to 0.003 of

an inch thick.

In vitro analysis

The in vitro release rate of chemical

into 50 ml of 37°C, pH 7 phosphate

buffer was determined by measuring the

concentration of naltrexone in the buffer daily. The sample was suspended in

the solution inside a Whatman extraction thimble. The 50 ml buffer solution

was changed weekly in order to maintain

the agent concentration below 20% of

saturation. The naltrexone base concentration was determined by means of a

Beckman LS100 liquid scintillator.

Riafluor ®

 (New England Nuclear) was

used as the cocktail. Naltrexone pamoate concentration was measured with a

Hilger-Watts spectrophotometer.

In vivo analysis

Male albino Charles River mice (18-22 g)

were used throughout the study. The

mice were anesthetized with Penthrane

(Abbott) and a small slit was made in

the scapular region. A single rod or

three beads were inserted below the slit

and the wound was closed by a silk

suture. The animals were allowed to

recover. Groups of ten animals were

then placed in cages and allowed to develop normally until the test date.

Ten individual mice were placed in separate metabolism cages for collection

of urine.

The urine was collected daily for the

duration of the experiment. The cages

were rinsed with distilled water and

the washings were added to the urine.

The combine and washings were urine

made up to 10 ml with water. A 1.0 ml

sample was taken and added to 10.0 ml

of Aquasol liquid scintillation cocktail (New England Nuclear) in a vial.

The radioactivity was measured using a

Beckman LS-230 liquid scintillation

counter. A quench curve was constructed

using a series of 3H quench standards

and this curve was used to correct all

counts.

The mouse tail flick procedure was used

throughout this study. A ten second

cut-off time was employed, and a control

reaction time of two to four seconds

was used. A rheostat, incorporated into

the instrument was used to adjust the

intensity of the light falling on the

tail of the mouse such that each reaction time fell within the stated range.

Animals with a control reaction time

outside the stated range were rejected.

The rheostat adjustment was only made

if a significant proportion (more than

two out of every ten mice) of the

reaction times were outside the range

of two to four sec. Groups of ten mice

were medicated with an ED80 dose

(14.0 mg/kg ip) of morphine sulphate

intraperitoneally and re-exposed to the

noxious stimulus twenty minutes later.

The analgesic response was calculated

as the percentage of the maximum possible response time (Harris and Pierson,

1964). The following formula was used

for these calculations: (test-control/

14
10-control) x 100 = % maximum possible

effect.

Each group of animals was tested by the

narcotic antagonist procedure only once.

Groups were planned so that testing was

performed every 7 or 10 days.

At the termination of each experiment,

the animals were sacrificed and the

implant site was examined. The condition of the implanted material was noted

and signs of encapsulation, injection

or irritation.

RESULTS AND DISCUSSION

The objectives of this ongoing study are

to develop implantable, biodegradable

naltrexone - polylacticlqlycolic acid

matrix systems which will sustain the

delivery of chemical to a biological

system for both one and six months.

Primary to achieving these objectives

are the development of reproducible

polymerization procedures using pharmacologicallv suitable catalysts. the

fabrication of matrices from-polymers

which hydrolyze within specified times,

and the selection of a practical sterilization process. Also, as the study

proceeded, it was expected that a relationship between the in vitro and in

vivo performances of the implants would

be developed.

Experiments were performed to ascertain

the effects on chemical release rate

and duration of the

degree of chemical loading,

solubility of the chemical in

both the surrounding environment and the polymer,

surface area/weight ratio and

porosity of the matrix, and

hydrolysis rate of the polymer

(i.e., composition of the polymer).

Quantitative evaluations of most of

these parameters have been completed.

The degree of chemical loading and the

solubilities have been identified as

most influential on the performance of

the implant. The duration of chemical

delivery is inversely proportional to

the drug loading level and the solubility of the chemical in the surrounding environment. Thus, matrices

designed to meet the six-month goal are

prepared with naltrexone pamoate which

is less soluble in aqueous solutions

than naltrexone base. The duration is

directly proportional to the solubility

of the drug in the polymer. Chemical/

polymer solubility effects are especially important in this study - the base

form of naltrexone forms a solid solution with polylactic/glycolic acids.

This behavior permits the use of the

very water soluble naltrexone base for

long duration implants.

In other experiments it has been determined that dip-coating the implants

with a thin layer of polylacticlglycolic

acid reduced the initial surge of chemical from the surface of the implant

and extended the duration of delivery.

The releases of chemical from rods,

spheres, and micron-size particle matrices have been measured. As expected

the rate is directly proportional to

the surface area/unit weight of implant.

Based on the above findings, matrices

have been fabricated specifically to

meet the forementioned goals. These

systems have been tested in vitro and

in vivo. One matrix system, dip-coated

rods of 33% by weight naltrexone pamoate in 90 L(+)/10 polylactic/glycolic

acid, has proved antagonistic to morphine challenge for 180 days in mice

( 8 mg naltrexone pamoate/mouse).

Using the one-to-one in vitro-to-in

vivo relationships which has beenobserved throughout the study, it is

anticipated that this system will continue to release chemical at an antagonistic level for 20 more days.

Inspection of the implant site after

180 days found no signs of foreign body

reaction (i.e., no inflamation or

walling-off of the implant) and a rod

which crumbled when probed'with forceps.

It is concluded that the formulation of

these rods is ideal for a 6-month naltrexone delivery system.

With another system, uncoated beads of

33% by weight 3H-naltrexone base in

90 L(+)/10 polylactic/glycolic acid,

chemical releases were measured both

in vivo (urine of mouse) and in vitro

and significant levels of antagonism to

morphine challenge was realize; for

55-60 days. Plots of the cumulative

results are presented in Figure 1. The

daily levels of 3H in the urine reported

as percentage of total 3H implanted are

presented graphically in Figure 2. In

this test, the release rate approximated zero-order throughout a major

portion of the delivery period; the

one-to-one in vitro-to-in vivo relationship was clearly demonstrated: and

similar to other tests in this study

roughly 50 percent of the implanted 3H

15


was found in the urine. Since the

release duration is inversely proportional to loading, it is hypothesized

that an uncoated bead matrix loaded to

50% by weight with naltrexone base will

meet the 30-day goal.

The above two systems exemplify typical

sample evaluation procedures and

results. The samples were prepared

with polymers synthesized from lactic

and glycolic acids. Extensive study of

the polymerization has identified reactant purity and ambient atmosphere as

the most influential factors affecting

the molecular weight of the polymer.

Reactants with sharp melting points and

an oxygen and moisture-free atmosphere

(e.g., <1 mm Hg vacuum) are required.

Using infrared analysis, the compositions of the copolymers were determined

to be similar to the reactant ratio

charged to the reaction vessel. This

point is important to note since some

catalysts react preferentially with one

reactant over another. It has also been

concluded that the product is a random,

straight-chained polymer.

In anticipation of more intensive animal tests, and ultimately, clinical

trials, sterilization procedures have

been screened to select a method applicable to the polylactic/glycolic acid

matrix drug delivery system. Normal

methods such as autoclaving and high

temperature, dry heat cannot be used

since the temperatures are above the

melting point of the polymer. And because the implant is porous, ethylene

oxide should not be used because complete degassing cannot be ensured.

Therefore, radiation sterilization was

selected for evaluation. Several types

of irradiation (e.g., high energy electrons and gamma ray) are under evaluation.

SUMMARY

The utility of the implantable polylactic/glycolic acid matrix system as a

means of providing the sustained delivery of naltrexone for periods up to 200

days has been demonstrated. Concurrent

with drug delivery, the polymer hydrolyzes to non-toxic metabolites which

are naturally eliminated. No foreign

body reaction to the implants has been

observed in mice. A one-to-one correlation between in vitro and in vivo

performance (i.e., release of chemical)

has been established.

Pharmacologically suitable catalysts

have been used to reproducibly prepare

polymers under controlled reaction

conditions. Polymers have been characterized using standard techniques and

a relationship between intrinsic viscosity and weight average molecular

weight established. Irradiation has

been selected as the most practical

method of sterilization for polylacticl

glycolic acid implants.

18
DEVELOPMENT OF

INJECTABLE MICROCAPSULES

FOR USE IN THE TREATMENT

OF NARCOTIC ADDICTION

Curt Thios, Ph.D.

Department of Chemical Engineering,

Washington University,

St. Louis, Missouri 63130

Long-acting injectable narcotic antagonist

formulations represent a potentially useful

means of treating heroin addicts. Conceptually, the necessary slow release drug formulations can be prepared by microencapsulating

the antagonist and then simply injecting the

capsules. The capsule wall should cause the

capsules to release their payload slowly,

thereby providing prolonged antagonistic

action. This project is concerned with reducing the above concept to practice. The

overall goal is to synthesize injectable

microcapsules that will last one to six months

in vivo.

The concept of an injectable microcapsule is

relatively simple, but the capsules used

must meet a number of stringent requirements.

Naturally, a prime requirement is that they

release their drug payload slowly and perform

for four weeks or more in vivo. This requires

deposition of relatively defect-free capsule

walls around individual drug particles. The

coating used to form the microcapsule wall

must be biocompatible and bioabsorbable. The

rate of bioabsorption preferably must be

such that the capsule wall will be completely

absorbed within six to twelve months after

injection.

The capsules must be small enough to freely

pass through a 21 gauge needle. This requires capsules smaller than 150 to 200μ.

The capsules preferably will be injected as

a saline dispersion and the maximum volume

of capsule dispersion administered as a single injection should be one to two cc.

In an effort to prepare capsules that meet

the above requirements, several new encapsulation procedures have been developed.

D1-poly(lactic acid) (d1-PLA) and lactide/

glycolide copolymers are used to form the

capsules because they are established biocompatible and bioabsorbable polymers. Narcotic

antagonists encapsulated successfully include

cyclazocine free base, naltrexone free base

and naltrexone pamoate. Active contents of

most capsules isolated range from 50 to 75

wt. percent. At least 40 to 50 percent of

the capsules isolated from a given capsule

batch are less than 300μ diameter; in several cases, up to 85% of the capsules isolated from a given batch are below 180μ

diameter. Although capsules with lactide/

glycolide coating have been made, thus far

capsules which release their antagonist payload the slowest have a d1-PLA wall.

Capsule quality in vitro is established by

extracting the capsules in a rotating bottle

extractor (37°C; pH 7.4 phosphate buffer)

and spectroscopically assaying the extracts

for the amount of antagonist released. Release curves obtained in this manner are not

zero order. The capsules consistently show

19
a relatively high initial rate of antagonist

release which declines steadily as extraction

continues. This behavior is attributed to

a distribution in quality of the capsules

being evaluated. Scanning electron micrographs of capsule surfaces reveal that the

walls of some capsules contain numerous

macroscopic defects like craters and pit

holes while other capsules are relatively

free of such defects. It is hypothesized

that the capsules with numerous defects release their payload rapidly upon immersion

in the extraction medium thereby yielding

the observed burst of antagonist release.

Once the defective capsules are empty, antagonist release continues as a steadily declining rate from capsules with progressively

fewer coating defects.

Support for the above hypothesis is provided

by a recent advance in encapsulation technology. This advance enables one to isolate

high yields of <180μ spherical capsules.

When viewed under a scanning electron microscope, the walls of such capsules appear to

have substantially fewer defects than previous capsules. The latest capsules release

naltrexone free base over more than a twoweek period in vitro and appear to be the

best capsules made to date. In vivo evaluations are now in progress, so their in vivo

lifetimes are unknown. Nevertheless, it is

relevant to note that the best previous

naltrexone pamoate capsules had an in vitro

lifetime of about two weeks. They provided

60 to 90% antagonism in vivo for 28 days

-.- after injection when injected into mice

through an 18 gauge needle as a peanut oil/

aluminum monostearate suspension at a 40 mg

antagonist/kg mouse dose level. Percent

antagonism was established by the mouse

tail-flick test procedure with 10 mg morphine/kg mouse as the challenge dose.

Significantly, the above in vivo experiment

was not terminated because the capsules ceased

to function. Termination occurred because

only four groups of test animals were innoculated for the experiment and data points

were obtained 7, 14, 21 and 28 days after

injection. The capsules provided 80% antagonism on day 28 after injection, so it is

clear that they still were releasing naltrexone free base at that time. How much

longer the capsules would have been effective

is unknown. Current in vivo experiments are

being carried out for longer periods and are

designed to answer this question. The <180μ

naltrexone pamoate capsules isolated recently

are expected to last at least 30 - 35 days

and perhaps longer when injected as a peanut

oil suspension. The latest capsules also

are being injected as a suspension in saline

containing 0.1 wt.% methylcellulose. The

ultimate goal is to eliminate the methylcellulose entirely and simply use saline.

Naltrexone free base is far more watersoluble than naltrexone pamoate. Thus, it

has been more difficult to synthesize naltrexone free base capsules capable of releasing their payload over a prolonged

period. The best naltrexone free base capsules made to date provide 75 to 95% antagonism for 14 days after injection. The injection medium was peanut oil/alminum

monostearate; the dose administered was 40 mg

antagonist/kg mouse. Recent encapsulation

work has focused exclusively on naltrexone

pamoate, so it is not known yet if the latest

encapsulation technology will yield longerlasting naltrexone free base capsules.

20
LONG-ACTlNG NARCOTIC

ANTAGONIST COMPLEXES

Allan P. Gray, Ph.D.

William J. Guordina, Ph.D.

IIT Research Institute

INTRODUCTION

The overall goal of this research is the

developmnt of improved drug therapy for the

treatment of narcotic dependence.

Narcotic antagonist drugs are of potentiai

value for this purpose but suffer fran the

drawback of being too short-acting. The

current consensus is that durations of the

order of atleast l-3months are desirable.

Although such durations would not be acceptable for narcotic substitutes such as

methadone, renewed interest has recently

been expressed in methadone preparations

effective for periods of several days.1

Our approach to achieving the goal has involved the preparation, in vitro and in vivo

study of sparingly soluble salts and ioniccomplexes of potentially useful drugs since

there are mple precedents in other

therapeutic areas showing that intramuscular

injection suspensions of such preparations

can provide slow release of drug and a useful

prolongation of action (Gray, Robinson, 1972;

1974; Gray, 1974). Our earlier work, which

has been reviewed (Gray et al., loc. cit.),

cocentrated on complexes of narcotic

antaganist drugs (cycclazocine, naloxone,

naltrexone and, more recently, diprenorphine). We are, however, currently turning

some of our attention back to the study of

related complexes of the narcotic replacement

drug, methadone.

Two of our preparations, naltrexone zinc

tannate (Gray, Robinson, 1974) and naltrexone aluminum tannate (Gray, 1974) appear

most promising. Administered in an alunium

monostearate gel suspension by intramuscular

injection to mice, antagonist activity

remains at a constant high level and then ,

drops off rapidly at the end of the period

(Gray et al. loc. cit.; Reuing2

). Durations

of activity in miceof 3-4 weeks have been

observed. Toxicological studies have thus

far revealed no contra-indicative effects.3

The pharmacokinetics of these preparations

are being evaluated,2 as well as their

effects in addicted monkeys4

.

21
REVIEW OF EARLIER WORK

We have evaluated the ability of close to 100

mono- and polybasic organic acids to form

water-insoluble salts with methadone and one

or the other of the narcotic antagonist drugs,

cyclazocine,nalcmone,naltrexoneand,more

recently, diprenorphine. About half the

acids yielded insoluble salts. Polybasic

acids affording insoluble salts were

evaluated for their ability to form drug:

metal; acid complexes with the polyvalent

metal ions, Zn++, Al++,Mg++and Ca++.

Optimum conditions for forming complexes have

been developed and the consistencyoftheir

composition has been established.

Salts were analyzed spectrophotometrically for

drug content, and complexes were analyzed for

drug and metal content. The in vitro degree

of dissociation at equilibrium was determined

for the preparations-suspended in a simulated

physiological buffer, pH 7.3, at 37°.

RECENT IN VITRO STUDIES

The work reported here has been carried out

in the period January l-April 15, 1975.

Naltrexone Complexes

Additional 15,16-3H-naltrexone aluminum

tannate (6) was prepared. 15,16-3Hnaltrexone base, specific activity 47.7

mCi/mg,5 was diluted in methanol solution

with cold naltrexone hydrochloride to give

15,16-3H-naltrexone hydrochloride, specific

activity 0.63 μCi/mg.

To a stirred aqueous solution of 360 mg

(10.6 meq) of tannic acid neutralized with

10.6 meq of 0.5 N NaOH was added 10.6 mq

of a freshly prepared 3 N aluminum nitrate

solution followed by an aqueous solution of

200 mg (5.3 mmole) of the 15,16-3H-naltrexone

hydrochloride. The mixture was stirred for

one hour, allowed to stand for 16 hours and

filtered. The water-washed precipitate was

dried to constant weight in vacuum, first at

room temperature and then at 60°. A yield

of 3.14 g of complex was obtained having

the following analysis:

% Naltrexone 19.0% (spectrophotometric assay)

% Aluminum 2.4% (Eriochrome cyanine R

dye method)

% Dissociation 10.1% (at equilibrium in

physiological buffer, pH 7.3, at

37°)

Preparations showing relatively low degrees

of dissociation in vitro, since it early

appeared that a high of dissociation

contraindicated a prolonged duration of

pharmacological action, were evaluated in

mice after intramucular administration at

several dose levels by the mouse tail-flick

test for the potency and duration of their

morphine antagonist activity.

Cur most promising preparations to date,

showing the most prolonged durations of

action without evidence of gross toxicity, have

been zinc and aluminum tannate comlexes. In

particular, the zinc and aluminum tannate

complexes of naltrexone are undergoing detailed evaluation as potential clinical candidates. Thus far, the most useful of several

injectable dosage forms studied is a

suspension in an aluminum monostearate gel in

peanut oil. The particle size of these complexes in suspension has been shown to be

distributed predominantly in a range below

16μ.

Because of the interest in the naltrexone

complexes, zinc and aluminum tannates of

radiolabeled (15,16-3H) naltrexone5 have

been prepared and are being evaluated for

Maintenance of blood and urine levels and 

at Ohio State University.2

for their pharmocokinetic behavior here and

Specific activity 0.7 μCi/mg drug base

(by scintillation counting).

Naltrexone preparations supplied for outside

evaluation during this period are listed in

Table 1.

Additional supplies of naltrexone zinc and

aluminum tannate complexes have been prepared and analyzed.

Methadone Complexes

Methadone zinc tannate and methadone aluminum tannate were prepared by the same

method (method 6) described for naltrexone

complexes by use of 2 equivalents of

neutralized tannic acid and 2 equivalents of

metal salt (zinc sulfate or aluninum

nitrate) per mole of methadone hydrochloride.

22
Table 1

Lot

Date Naltrexone Complex No Wt, g

3/4-3/18/75 Zinc tannate (6) C21885 750 mg

E-8-20

3/4-3/24/75 Aluminum tannate (6) C21950 750 mg

E-10-21

3/4/75 Zinc tannate (6) C22089 2.5 g

E-14-13

3/4/75 Aluminum tannate (6) C22040 6.6 g

E-12-10

4/16/75 15,16-3H-labeled C22142 1.25 g

Aluminum tannate (6) E-16-17

Recipient Study

D. A. McCarthy Addicted

Parke-Davis monkeys

D. A. McCarthy

Parke-Davis

C. W. Mastri

Industrial Biotest

C. W. Mastri

Industrial Biotest

R. H. Reuning

OSU

Addicted

monkeys

Toxicology

Toxicology

Pbarmacokinetics

Analytical data are given in Table 2.

Table 2

Zinc Tannate Aluminum Tannate

% Methadone 31.9 27.6

% Metal 4.7a

2.0

% Dissociation 4.1 4.3

aBy atomic absorption.

To assist in the in vivo evaluation of

methadone complexes,their radiolabeled

counterparts have been prepared. o,o'-3HMethadone base, specific activity 53.9 mCi/mg,5

was diluted in ethanol solution with cold

methadone hydrochloride to give o,o'-3Hmethadone hydrochloride, specific activity

1.2 μCi/mg. o,o'-3H-Methadone zinc tannate

and aluminum tannate complexes have been

prepared fran the tritium-labeled hydrochloride

by method 6; analysis is not yet complete.

Thin-Layer Chranatography of Radiolabeled

Hydrochloride Salts

The purity and stability of 15,16-3H-naltrexone hydrochloride and o,o'-3H-methadone

hydrochloride are being studied by thin-layer

chromatography. After experimentation with

the cold salts, identifying spots by UVfluorescence and by various spot reagents,

we have elected to work primarily with the

following two solvent systems:

a) chloroform:methanol:ammonia (90:10:1)

b) n-butanol:acetic acid:water (60:15:30)

With the tritim-labeled bases and hydrochloride salts, chromatograms were developed

on Polygram Sil G sheets. The sheets were cut

into l-cm-wide strips and the radioactivity

of each strip was determined by scintillation

counting. Significant amounts of impurities

were noted in the radiolabeled hydrochloride

samples. Further investigation is in progress.

Dissociation Under Simulated Sink Conditions

It had previously been noted that when an

amount of naltrexone zinc or alluminum tannate

complex calculated to contain 15 mg of drug

base was magnetically stirred in 10 ml of

isotonic phosphate buffer, pH 7.3, at 37°,

equilibrium dissociation of the complex was

attained in less than 15 minutes. Therefore,

in order to simulate sink conditions, the

following procedure was adopted.

An amount of complex calculated to contain

15 mg of naltrexone base was stirred in 10 ml

of buffer, pH 7.3, for 15 minutes at 37+0.l°.

The mixture was centrifuged, an 8-ml aliquot

of the supernatant removed and replaced by 8

ml of fresh butter. The process was repeated.

The concent ration of naltrexone in each

removed aliquot was determined spectrophotometrically in 0.1 N hydrochloric acid in the

usual way.

23
Results for naltrexone zinc tannate are shown The amount of drug remaining in the residual

in Figure 1. A total of 10.7 mg, approximate- complex was determined spectrophotometrily 70%, of the naltrexone had been removed cally to be 2.3 mg.

by 26 portions of buffer. Although, as can be

seen from Figure 1, the amount of drug re- For comparison, a parallel experiment is

leased with each portion of buffer was not being carried out with naltrexone aluminum

constant, it does follow a smooth curve which tannate.

appears to be approaching zero release

asymptotically.

RECENT IN VIVO STUDIES

Diprenorphine Complexes

ED80 Determinations

The ED80 dosages for diprenorphine HC1,

diprenorphine zinc tannate, and diprenorphine aluminum tannate in peanut oil

are 0.03 mg/kg, 0.048 mg/kg, and 0.092 mg/kg,

respectively. The antagonistic effect of

diprenorphine to the analgesic effect of

morphine (4Omg/kg/ip) on the tail flick

test was measured 40 minutes after the

intramuscular injection of diprenorphine and

30 minutes after the intraperitoneal

injection of morphine. The ED8O dosages for

the diprenorphine preparations are somewhat

24

greater than those for the long acting

preparations of naltrexone possibly because

diprenorphine is released at a slower rate

fromthe site of injection. The duration of

antagonism of analgesia by the ED80 dose

of diprenorphine hydrochloride is shown in

Table 3.

Antagonism of Analgesia

Table 4 shows the duration of the antagonism

to morphine of the long acting preparations

of diprenorphine (4 mg/kg in peanut oil)

compared to that of the hydrohloride.

The diprenorphine complexes exhibited

durations of action comparable to those of

the corresponding naltrexone complexes at
Table 3

ANTAGONISM OF ANALGESIA BY THE ED80 DOSE

OF DIPRENORPHINE

%Antagonism Produced by a Dose

Containing 0.03 Mg/kg. Drug at:

Drug Hours

(in peanut oil) 1 35

Diprenorphine HCl 82 64 24

Table 4

ANTAGONISM OF ANALGESIAa

Drug

(in peanut oil)

Diprenorphine HCl

%Antagdsm Produced by a Dose

Containing 4.0 mg/kg. Drug at:

Hours

94 95 83 16 - - -

2 4 5 24 48 72 96

Diprenorphine Zinc

Tannate - - 98 63 14 19 -

Diprenorphine Aluminum

Tannate - - 97 78 64 42 16

aMorphine 40 mg/kg/ip injected 30 min before the tail flick

test.

this dose level in peanut oil. Diprenorphine appears to offer no advantage in

duration of action and, moreover, exhibits

definite agonist properties.

Methadone Complexes

Measurement of the duration of action of long

acting preparations of methadone is being

attempted by radiolabeling as well as pharmcologicaltechniques. In one series of

experiments which will begin shortly, radiolabeled methadone HC1, methadone zinc tannate,

or methadone aluminum tannate will be injected intramuscularly in mice. The plasma

and brain tissue levels of radioactivity from

these injections will then be measured over

several days.

In another series of experiments, methadone

HCl, methadone zinc tannate, ormethadone

aluminum tannate (4O mg/kg methadone base)

is substituted for morphine in morphine

dependent mice. Mice are addicted to

morphine by a series of intraperitoneal

injections of morphine given at one hour

intervals as described (Iorio et al., 1975).

The following five doses of morphine are

administered in ascending order: 12.5, 25,

50, 50, and 50 mg/kg/ip. An intramuscular

injection methadone preparation is substituted for morphine at the sixth injection.

The duration of the addiction in these mice

is tested by the presence of jumping

precipitated by naloxone (25 mg/kg/ip).

Groups of mice were tested at 1, 24, 48, and

72 hours after the intramuscular injection.

Mice receiving the vehicle (peanut oil) or

methadone (40 mg/kg) jumped at nearly the

same rate following an injection of naloxone

(25 mg/kg) at each of the test periods.

These studies are being continued.

In another group of experiments, mice were

addicted to morphine with injections given

twice daily for two weeks; they received

morphine (150 mg/kg/ip) on the last two

days. These studies showed that methadone

(4O mg/kg) zinc tannate in peanut oil

vehicle prolonged naloxone (10 mg/kg/ip)

induced jumping by two days as compared to

vehicle and methadcne HC1 treated mice.

25
Radiolabeled Naltrexone

3H Naltrexone HC1 (4mg/kg) in the aluminum

monostearate vehicle was injected intraumscularly in mice. Mice were sacrificed by

decapitation 1, 2, 4, and 24 hr after the

injection. The brains of the mice were

removed and immediately digested in NCS tissue

solubilizer. The plasma fraction of the

blood was separated via centrifugation. The

levelofradioactivity in each sample as

measured 24 hrs later (see Table 5).

The plasma levels of radioactivity following

an intramuscular injection of naltrexone HCl

in an aluminum monostearate gel vehicle

decreased steadily over the 24 hr sampling

period. In contrast, the brain levels of

radioactivity did not decrease during the

same period of time. These results suggest

that the plasma levels of naltrexone do not

directly relate to the brain levels of

naltrexone. The work is being continued.

Table 5

PLASMA AND BRAIN LEVELS AFTER INTRAMUSCULAR INJECTION

IN MICE OF RADIOLABELED NALTREX0NE HC1

IN ALUMINUM MONOSTEARATE GEL VEHICLE

(Hours)

Level of Radioactivity in Plasma and Brain

as ng/ml and ng/g, Respectively,

Time 0f Naltrexone

Plasma Brain

1 408 86

2 380 77

4 200 79

24 128 a

48 a a

aExperiments in progress.

FOOTNOTES REFERENCES

1. R. E. Willette, personal communication.

2. R. H. Reuning, NIDA Progress Reports,

persona1 communications.

3. C. W.Mastri, personal communication.

4. D. A. McCarthy, Parke-Davis.

5. Obtained from J.A. Kepler, Research

Triangle Institute.

Gray, A. P., Conference on Biomedical

Research in Narcotic Abuse Problems,

Vancouver, B. C., November 1974; Proceedings

in press.

Gray, A. P. and Robinson, D. S., First

International Conference on Narcotic

Antagonists, Airlie House, Warrenton, Va.,

Nov. 1972; Proceedings: Narcotic

Antagonists, M. C. Braude, L. S. Harris,

E. L. May, J. P. Smith and J. E.

Villarreal, Ed., Advances in Biochemical

Psychopharmacology, Vol. 8, Raven Press,

New York, 1974, pp: 555-568.

Gray, A. P. and Robinson, D. S., J. Pharm.

Sci., 63, 159 (1974).

Iorio, L. C., Deacon, M. A. and Ryan, E. A.,

J. Pharmacol. Exptl. Therap., 192, 58 (1975).

26
SUSTAINED RELEASE OF

NALTREXONE FROM

GLYCERIDE IMPLANTS

M.F. Sullivan Ph.D.

D. R. Kalkwarf, Ph. D.

Battelle, Pacific Northwest

Laboratories

INTRODUCTION

This research is concerned with developing injectable and biodegradable

systems that release the narcotic

antagonist, naltrexone, in vivo for

at least 30 days at pharmacologically

effective rates. Such preparations

should improve the efficiency of

treatment of heroin addicts with

naltrexone by compensating for the

rapid metabolism of this antagonist.

Natural glycerides that are solids at

body temperature yet can be melted

and molded at relatively low temperatures appear to be appropriate

vehicles for delivering the drug at

effective and constant rates. These

substances provide a reservoir from

which the drug can diffuse gradually

during treatment and that is itself

eventually absorbed by the body with

negligible toxic effects, This report describes their utilization in

naltrexone-delivery systems.

DEVELOPMENT AND TESTING PROCEDURES

Implant preparation:

Small pellets and particles were prepared containing naltrexone dispersed

homogeneously in various glycerides.

Crystals of naltrexone (diameter <125μ)

were added with vigorous stirring to

molten combination of a-monopalmitin,

1,2- and 1,3- dipalmitin and tripalmitin at 80 C. To form pellets,

the liquid suspensions were drawn -into cylindrical molds, 1.7-mm in

diameter, and cooled rapidly until

they solidified. The cylinders were

extruded, cut into 3-mm lengths for

use in mice and into 10-mm lengths for

use in rats, rabbits, monkeys and

swine. Particles were prepared in two

size ranges, 125-250 μ and < 125 μ by

grinding and sieving the solid dispersions, They were then suspended

in an aqueous solution containing

1.5% methyl cellulose at a concentration that would allow injection of

the desired amount of naltrexone in

a 0.25 ml volume.

In Vitro Evaluation

The release rates of naltrexone from

the preparations were measured in

vitro to determine if the release

characteristics justified further

testing in vivo. The in vitro

evaluation was done by placing a

sample in a continuous flow of iso27
tonic saline solution at 37 oC using from the interior, The rate of drug

the apparatus shown in Figure 1. release into this "perfect sink" was

The flow rate was increased until determined from spectrophotometric

naltrexone was removed from the measurements on the effluent solusurface as fast as it diffused there tion at 281 nm,

FIGURE 1. CONTINUOUS FLOW SYSTEM FOR DETERMINING DRUG RELEASE

RATES IN VITRO

In Vivo Evaluation according to the procedure of D'Amour

and Smith, 1941 as modified for mice

The mice used in these studies were by Harris et al., 1969, At least six

of the Swiss-Webster strain weigh- antagonist-implanted mice, two

ing between 20-30 gm, Experimental

and control animals were maintained

in groups of 15 and fed Purina mouse

pellets ad libitum. Their weights

were determined weekly, The in vivo

vehicle controls and two normal controls were used for each test, unless

otherwise specified,

At the conclusion of the pharmacologic

measurement, the mice were anesthetized, the pellets were removed, and

the implantation sites were inspected

for any gross signs of tissue incompatibility. Residual naltrexone in

each pellet was determined spectrophotometrically after dissolving it

in chloroform. Residual glyceride

was determined by esterifying a

sample of the chloroform solution

with methyl alcohol in the presence

of boron trifluride and determining

the amount of ester with a gas

chromatograph.

measurement of the pharmacologic

effectiveness of the various preparations was done by either implanting

pellets subcutaneously into the nape

of the neck with a 12-gauge trocar

or by injecting a suspension of

naltrexone loaded particles in 1.5%

methylcellulose as the suspending

agent. Both vehicle-implanted and

normal mice were used as morphineinjected controls, A dose of 15 mg/kg

of morphine sulphate was injected

subcutaneously and the tail-flick response was measured 30 minutes later

28
FIGURE 2. RELEASE OF NALTREXONE FROM 1.7 x 10.0 mm PELLETS

CONTAINING 30% NALTREXONE IN 25:75 DIPALMITIN:

TRIPALMITIN.

FIGURE 4. RELEASE OF NALTREXONE IN MICE FROM A 1.7 x 3.0 mm

PELLET CONTAlNlNG 30% FREE BASE IN 25:75 DIPALMITIN:

TRIPALMITIN AND COMPARISON WITH THE REACTION TIME

OF THE MICE IN THE TAIL-FLICK TEST.

FIGURE 3. RELEASE OF NALTREXONE IN MICE FROM A 1.7 x 3.0 mm

PELLET CONTAINING 0.3% FREE BASE IN 25:75 DIPALMITIN:

TRIPALMITIN AND COMPARISON WITH THE REACTION TIME

OF THE MICE IN THE TAIL-FLICK TEST.

FlGURE 5. RELEASE OF NALTREXONEAND RESULTING

ANTAGONISTIC EFFECT FROM A 1.7 x 3.0 mm

PELLET CONTAINING 30% NALTREXONE (FREE

BASE IN DIPALMITIN AND IMPLANTED S.C.

IN MICE

29
RESULTS

An early preparation that we evaluated was a solid implant, 1.7 x 10

mm, that was loaded with 30 percent

naltrexone. The pellet matrix was

dipalmitin:tripalmitin in the

ratio of 25:75. The release rate

into isotonic saline at 37 C under

"perfect sink" conditions is

shown in Figure 2 along with the

in vivo release that occurred from

subcutaneous implants that were

removed in groups of 3 at intervals

after treatment. The release rate

of naltrexone in vitro gradually

decreased with time as its concentration near the surface diminished;

however, the release rate in vivo

remained constant, indicating that

biodegradation had continually exposed fresh surface, Rats that were

implanted with a single pellet were

unaffected by morphine for 33 days

after treatment as judged by their

tail-flick response.

Small pellets (1.7 x 3.0 mm) were

implanted in mice to determine the

effect of the naltrexone dose on

the pharmacologic response to

morphine analgesia. The naltrexonerelease rate and the duration of

action of the implants is partially

dependent on the concentration of

the antagonist in the glyceride

matrix. This can be seen in

Figure 3 and 4 where a 0.3% naltrexone loading resulted in the release

of 2.0 μg/day. However, both

preparations were ineffective for

antagonizing morphine beyond 2 weeks

after they were implanted.

The duration of naltrexone effectiveness by a single injection is

extended when it is administered as

naltrexone pamoate. Pellets containing various combinations of dipalmitin and tripalmitin along with

naltrexone pamoate were tested for

their release characteristics in

vitro. Naltrexone pamoate was

released from a dipalmitin-tripalmitin matrix more rapidly than when

it was present as the free base,

This formulation appeared to be unsuitable for providing 30-days of

effective antagonism.

Pellets loaded with 30% naltrexone

in an equal mixture of 1,2- and

1,3- dipalmitin were tested both

in vitro and in vivo and were found

to release the antagonist at pharmacologically effective rates for 30

days, This is shown in Figure 5.

Three of the ten mice tested at each

time interval were implanted with

pellets containing 3H-naltrexone.

Urine collected daily from these

animals during the 2 days prior to

the tail-flick test showed that

urinary excretion of tritium paralleled the in vitro release of

naltrexone as shown in Figure 6.

Both types of data show that the

release rate of naltrexone was high

initially but decreased rapidly to

a constant value, Our earlier

studies with 3H-naltrexone showed

that approximately 50% of the dose

could be recovered from the urine

of mice after administration by

single injection or by continuous

infusion. It was on this basis

that the quantity of naltrexone

metabolized daily was calculated.

The dis appearance of drug and

dipalmitin from 1.7 x 3.0 mm pellets

is shown in Figure 7. Earlier tests

showed that 23 of 29 1.7 x 10 mm

dipalmitin implants disappeared completely from intramuscular sites in

the buttocks of rabbits within 3

months. The data points in Figure

7 are not sufficient to predict the

disappearance rate of the 1.7 x

3.0 mm pellets in mice accurately,

but a line is drawn through them to

present a "best estimate" of the

biodegradation profile.

In an effort to obtain pellets that

biodegraded more rapidly, glyceride

mixtures containing either 10% -

monopalmitin and 90% dipalmitin or

20% a-monopalmitin and 80%-dipalmitin were loaded with 30% naltrexone

and tested, The biodegradation

rates of the preparations did appear

to be more rapid, but the release

rates of naltrexone were also greater, as shown in Figure 8. Consequently, the drug contents of the

pellets were depleted within a few

days.

In a further effort to obtain effective preparations that biodegraded

more rapidly, particles containing

30% naltrexone dispersed in either

dipalmitin or tripalmitin were prepared and tested. Suspensions of

these particles in aqueous methyl

cellulose solution were injected

subcutaneously in mice and tested

at intervals thereafter for their

antagonist potency by the tail30
FIGURE 7.

FIGURE 6. COMPARISON OF NALTREXONE RELEASE RATES

FROM A PELLET IN VITRO AND AS DETERMINED

BY URINARY EXCRETION OF 3H-NALTREXONE

FROM MICE. PELLETS WERE 1.7 x 3.0 mm IN

SIZE AND CONTAINED 30% NALTREXONE IN

DIPALMITIN

LOSS OF NALTREXONE AND GLYCERIDE FROM

A 17 x 3.0 mm, 7.8 mg PELLET IMPLANTED

SUBCUTANEOUSLY AND CONTAINING 30%

NALTREXONE (FREE BASE) IN DIPALMITIN

FIGURE 8.

FIGURE 9. EFFECTIVENESS OF 8.0 mg OF TRIPALMITIN

( )OR DIPALMITIN ( ) PARTICLES

OF DESIGNATED SIZE CONTAINING 30%

NALTREXONE FOR ANTAGONIZING MORPHINE

(15 mg/kg) USING THE MOUSE TAIL-FLICK

TEST

COMPARISON OF IN VIVO AND IN VITRO

RELEASE OF NALTREXONE FROM 1.7 x 3.0 mm

PELLETS CONTAINING 30% NALTREXONE

(FREE BASE) IN DIPALMITIN: MONOPALMITIN MIXTURES, 100.0, 90:10, OR 80:20

31
flick response. The results are

shown in Figure 9. This preliminary evaluation in only a few mice

showed that particles less than

125 μ and dipalmitin particles were

relatively ineffective for any

prolonged duration of action. On

the other hand evaluation in at

least 6 mice per interval showed

that tripalmitin particles in the

125-250 μ range provided at least

one month of antagonist activity.

Further refinement of these particles is now in progress to provide

a more uniform glyceride coating

that may extend the life of this

preparation.

No appreciable tissue response was

found after implantation of either

dipalmitin or tripalmitin in rats,

rabbits, or swine at intramuscular,

subcutaneous, or intraperitoneal

sites. No significant amount of

fibrosis was seen as long as 1 year

after treatment with the slower

biodegrading glycerides. There has

been no gross indication that any

of our more recent preparations are

toxic.

REFERENCES

1. D’Amour, F. E. and Smith, D. L.

A Method for Determining Loss of

Pain Sensation, J. PharmacoL. and

Exptl. Therap. 72, 74-79, 1941.

2. Harris, L. S., Dewey, D. L., Howes,

J., Kennedy, J. S., and Pars, H.

Narcotic Antagonist Analgetics,

Possible Cholinergetic Mechanisms.

J. Pharmacol and Expt'l Therap.

169, 17-22, 1969.

CONCLUSIONS

These studies have shown that

glycerides and naltrexone can be

combined to regulate the release of

that antagonist in-laboratory

animals for periods of at least one

month. The dose released is dependent on the drug concentration within the matrix of the release vehicle and on the glyceride composition of the matrix itself. The rate

of biodegradability of a solid implant can be increased by increasing the proportion of the lower

molecular weight glyceride in the

matrix. However, this change also

speeds up the drug release rate

thereby limiting the usefulness of

this manipulation. Hopefully, the

biodegradation rate may also be

increased by administering slowrelease vehicles in particle form

due to the increased surface area.

This form of the vehicle would be

less retrievable by an addict

that wished to remove the implant

and may be preferred by the

therapist. Our current efforts are

directed toward improving both the

particles and the pellet implants in

order to offer a range of slow release devices that can be chosen to

fit the needs of the addict being

treated with narcotic antagonists.

32
USE OF SYNTHETIC POLYPEPTIDES

IN THE PREPARATION OF

BIODEGRADABLE DELIVERY VEHICLES

FOR NARCOTIC ANTAGONISTS

Kenneth R. Sidman

Douglas L. Arnold, Ph.D.

William D. Steber

Lita Nelsen

F.E. Granchelli, Ph.D.

Suresh G. Sheth

ARTHUR D. LITTLE, INC.

INTRODUCTION

Under contract No. ADM-45-74-218 we have been

working on the development of practical,

clinically acceptable narcotic antagonist

delivery vehicles prepared from synthetic

polypeptides such as glutamic acid/leucine

copolymers. The synthetic polypeptides were

chosen because they potentially exhibit

diffusional release rates and biodegradation

rates in the regions of interest, and have

been shown to be biocompatible. Two types of

systems are being sought: One capable of

administering naltrexone for one month and

another for approximately six months. It is

desired that these systems biodegrade at a

sufficiently rapid rate that they are essentially "absorbed" as soon after they are

exhausted of antagonist as possible.

In addition, in view of the constantly evolving concepts advanced in the areas of antagonist development, psychopharmacology, and

protocols for patient treatment, it is

important that the delivery system developed

33

in this program be amenable to other antagonists, and be capable of being fabricated into

a variety of dosage forms, including integral

shapes such as rods and fibers, as well as

discontinuous forms such as microcapsules and

microbeads.

Although the antagonist may be released from

the matrix by diffusion, by the degradation

of the matrix, or by a combination of both

mechanisms, we are pursuing the development

of a system which releases naltrexone essentially

by diffusion alone because, at this stage, such

a system can most easily be fabricated. Biodegradation subsequently removes the implant matrix.

METHODS

Preparation of Polymers

The glutamic acid/leucine (Glu/Leu) copolymers

were prepared following the procedure of Fasman

et.al. (Fasman, 1964). The polymers were charac-
terized by infrared spectroscopy, ultraviolet

spectroscopy, intrinsic viscosity, and solubility

in N, N dimethy l formamide(DMF).

Preparation of Delivery Vehicles

Film-shaped implants were prepared by casting a

DMF solution of the copolymer and naltrexone

(labelled with tritium and exhibiting a specific

activity of 0.48 mCi/gram) into a teflon mold.

By alternately casting and drying (at 40°C-60°C)

thin layers of the solution, films 0.007cm to

0.07Ocm thick were built up. The films were then

placed in a vacuum oven at 50°C-70°C, 30 in. Hg

vacuum for twenty-four hours to remove the last

traces of solvent. Individual devices exhibiting

total surface areas of one to two cm were cut

from the films.

Rod-shaped implants 0.04cm to 0.08cm in diameter

were prepared by extrusion through a die. Typically, finely ground polymer was dry-blended in the

desired proportion with the tritium-labelled

naltrexone. Sufficient solvent was added to make

a 50% solids paste. This was then placed in a

steel compression mold that had a O.lcm diameter

hole bored in its side (later the mold was modified by drilling and tapping so that it would

accept a screw-in die). Rod was extruded at room

temperature by exerting moderate pressure on the

plunger. The rod was then dried to remove the

solvent.

Tube devices with an inside diameter of 0.18cm

and wall thicknesses ranging from 0.0026cm to

0.012cm were prepared by casting a solution of

the polymer onto a rotating glass mandrel. Extruded naltrexone/polymer cores consisting of 80%

naltrexone by weight were inserted into the tubes.

Pure polymer plugs were then inserted and cemented

into the ends of the tubes.

In Vitro Evaluation of Naltrexone Release Rates

The delivery vehicles were rinsed briefly with

Farle's Balanced Salt Solution (IX - without

phenol red, Grand Island Biological Company)and

were placed in stoppered test tubes containing

10 ml of the Farle's solution. The tubes were

placed in a shaker water bath at 37°C and were

gently agitated. Drug concentration of the

solution was periodically assayed by scintillation

counting techniques to permit an evaluation of

the in vitro drug-release behavior of the copolymers. (The solutions were replaced sufficiently

frequently that the naltrexone concentration

remained far below saturation, i.e. below

460 mg/100 ml).

Charles River CD #l mice were prepared for device

evaluation by implanting the delivery vehicles

using the following protocol:

IN VITRO EVALUATION

Surgery: Under pentobarbital Na(60 mg/kg) anesthesia the drug release devices were implanted

subcutaneously on the back of a mouse.

Apparatus: Immediately after surgery, the mice

were fitted with coprophagy cups and placed in

metabolic cages. The coprophagy cups assured

complete separation of urine and feces and thus

allowed for an accurate evaluation of the relative

magnitudes of excretion by both routes. Urine

and feces were collected daily for the first

week. and two days per week thereafter.

The wire floors and draining portions of the

cages were flushed to dissolve dried urine and

assure a thorough removal of radioactivity from

the cages. Feces samples were removed from the

tail cups and held in air-tight vials until

analyzed.

Analysis of Excreta: Small aliquots of diluted

urine (urine and rinse) were analyzed directly

by scintillation methodology. Radioactivity in

the feces was determined by homogenizing the

samples and then analyzing them using scintillation counting procedures.

Tail Flick Bioassay: A Dewey-Harris tail-flick

apparatus constructed by Dr. William L. Dewey

and his associates was used in this program.

Experimental animals were weighed and control

tail-flick times were determined prior to any

treatment or manipulation. Animals whose tailflick times were outside the range of 2-4 seconds

were discarded. At various times after implantation, test tail-flick times were taken thirty

minutes following an AD80 dose of morphine -

SO4 (17.0 mg/kg).

RESULTS

Films

Initially, model delivery systems in film form

were prepared using various naltrexone/polypeptide compositions to assess the compatibility of

the antagonist in the polymer. Naltrexone was

found to be highly compatible with Glu/Leu

copolymers, showing no tendency to bloom to the

surface of films containing at least 40% drug

by weight. This result indicates that the

34
naltrexone/polypeptide compositions are

capable of being stored for prolonged periods

of time without changing their composition or

structure.

Films prepared from 40/60, (i.e., 40 mole percent leucine/60 mole percent glutamic acid),

35/65, 20/80, and 20/80 Glu/Leu copolymers

containing 10% to 40% naltrexone by weight

were evaluated in vitro in mice. The naltrexone was found to be released by diffusion,

exhibiting a diffusion coefficient that varied

as a function of the copolymer composition and

the initial naltrexone loading. For example,

the diffusion coefficient increases as the

glutamic acid content is raised; and, at a

fixed glutamic acid content, it increases as

the initial naltrexone concentration is increased. A wide range in diffusion coefficients can be achieved (see Table 1). These

TABLE 1

35/65 10

20

40

20/80

10/90

values are within practical ranges of interest

for the development of delivery vehicles

capable of meeting the program goals.

The diffusion coefficients may be used to

predict release rate versus time behavior

using suitable equations. One useful expression for release rate is the following.

(Baker, 1974)

where D=diffusion coefficient, =thickness,

t=time

EFFECT OF COPOLYMER COMPOSITION AND ANTAGONIST LOADING

ON NALTREXONE DIFFUSION COEFFICIENTS

Copolymer

Composition

(Glu/Leu)

40/60

Naltrexone

Content

Wt. %)

10

20

40

20

40

40

Diffusion

Coefficient

(Cm2/Hr)

20 x 10-7

22 x 10-7

120 x 10-7

0.31 x 10-7

11.5 x 10-7

26 x 10-7

1.0 x 10-7

12 x 10-7

19 x 10-7

35

Time To Release

One-Half of Initial Drug Loading

From Films With Thicknesses

Of

0.007 Cm20.070 Cm2

1.2 Hrs

1.1

0.2

78 7800

2.1 210

0.9 90

24 2400

2.0 200

1.3

120 Hrs

110

20

130
Implant

Thickness

cm

0.061

0.056

0.048

0.046

0.043

0.043

Initial

Loading

Implant of

Drug

μg

5800

4900

6600

4050

4650

5500

TABLE 2

COMPARISON OF IN VIVO RELEASE BEHAVIOR OF 35/65

GLU/LEU FILMS CONTAINING 20% NALTREXONE

Percent Antagonism*

Mouse 1 4 7 9 11 14 16 18 21 24 28-Days

GT - - 100 - - 100 - - 100 100 -

GL - 73 64 - 65 - - 100 100 - 100

G H - - 100 - - 100 - - 100 100 -

RL 100 - - 100 - - 100 - 100 100 -

G R - 100 - - 100 - - 100 100 - 100

RT 100 - - 100 - - 72 - 100 - 100

*Mice were challenged with 17 mg/kg morphine sulfate

administered i.p., and were tested after 30 min.

36
TABLE 3

CHARACSTICS OF 20/80 GLU/LEU RODS

CONTAINING 40% NALTREXONE EVALUATED IN VITRO

Average Diffusion

Diameter Weight Sample No. Coefficient

cm mg cm

2

/hr

Uncoated Rods

0.061 7.5 18546-75-40-78-3 1.3 x 10-8

0.065 7.3 18546-75-40-78-2

Rods with Three Coats

0.060 6.1 18546-76-3-40-79-l 1.0 x 10-8

0.068 6.5 18546-76-3-40-79-3

Rods with Five Coats

0.056 7.1

0.064 6.9

28546-76-5-40-80-3

18546-76-5-40-80-l

0.38 x 10-8

37
With this expression, a comparison of the

theoretical release rate with the actual

in vivo release rates may be made. Figure 1

shows such a comparison for typical 35/65

Glu/Leu films containing 20% naltrexone by

weight. Here the actual urinary recovery of

radioactive drug and metabolites is plotted

versus time. (In these experiments the urinary

radioactivity accounted for 80-90% of the total

excreted, and, therefore, the urine data

provided a relatively accurate indication of

the rates of release from the implants). Note

that the actual release rates follow the theoretical predictions quite well, confirming that

naltrexone is released from films by diffusion.

Since a naltrexone dosage of l-2 μg/hr per

mouse is all that is necessary to block an

AD8o dose of morphine sulfate, animals receiving

these film implants were blocked for more than

28 days (see Table 2).

Rods

Solid rod shaped devices released naltrexone

in a manner similar to that observed with the

films, indicating that a prolonged release

can be achieved, albeit at a rate that continuously decreases with tine (as predicted by

standard diffusion equations).

Rods prepared from 20/80 Glu/Leu, containing

40% naltrexone by weight, were found to exhibit an unexpectedly high initial rate of

release, which could be substantially reduced

by overcoating the rods with pure 20/80 Glu/

Leu copolymer (see Table 3 and Figure 2). Rods

receiving five coats of polymer exhibited

diffusion coefficients that were constant with

time, and corresponding release rates that

were constant when plotted versus the square

root of tine. After approximately 70 days,

10% of the initial drug was released from

these devices.

Despite the prolonged release obtainable from

monolithic devices such as films and rods,

it is clear that constant rates of release

by diffusion cannot be achieved with these

dosage forms because the drug flux continuously decreases as the drug concentration

gradient within the device decreases.

Tube forms, however, behave quite differently.

REFERENCES

Tubes

Tube devices filled with a saturated solution

of the antagonist (with excess solid present)

will maintain the concentration gradient across

the tube wall at a fixed value. As a consequence, these devices are capable of delivering naltrexone at a constant rate over a

prolonged period (until the solid phase

disappears).

38

Preliminary experiments with tubes have begun.

In these early experiments, variations in

release rate were encountered because of poor

transport of naltrexone from the solid drug

core to the interior wall of the tube. As

previously stated, a constant release rate

depends on maintaining a saturated solution

of naltrexone in the interior of the tubes.

The tubes in these experiments were only

partially filled with liquid, resulting in

only partial utilization of the surface area

available for diffusive release.

We are currently studying techniques for prefilling tubes with saline so that a saturated

solution of naltrexone will be in contact with

the entire inner surface of the tubes. This will

ensure full utilization of the surface area for

diffusive release, and consequently higher and

more predictable release rates. Tubes with wall

thicknesses ranging from 0.026mm to 0.12mm are

being employed in this study so that an evaluation of the influence of wall thickness may be

made.

CONCLUSIONS

Synthetic polypeptides may be fabricated into a

variety of naltrexone delivery vehicles capable

of administering the antagonist for prolonged

periods of time. Tube dosage forms, in particular, offer advantages for this application since

They my contain drug loadings as high as

80% to 90% by weight, allowing a minimum overall

size of depot.

They would be expected to exhibit a constant rate of antagonist release (by diffusion),

and hence a high utilization of the drug payload.

They would contain a minimum amount of

polymer to be removed by biodegradation.

They may be administered by trocar injection.

However, additional work is required to develop

techniques for the preparation of vehicles that

release antagonist at a reproducible and constant

rate.

Raker, R.W. and Lonsdale, H.K., "Controlled

Release: Mechanisms and Rates," Controlled

Release of Biologically Active Agents,

Tbnquary, A.C. and Lacey, R.E. (Advances in

Experimental Medicine and Biology,. 47)

Plenum Press, New York, 41, 1974.

Fasman, G.D., Lindblow, C. and Bodenheimer, E.,

Biochemistry, 3, 155, 1964.
DEVELOPMENT OF CHRONOMERSTM

FOR NARCOTIC ANTAGONISTS

Richard C. Capozza, Ph. D.

Edward E. Schmitt, Ph. D.

Lee Richardson Sendelbeck, M.S.

ALZA Corporation

BACKGROUND

The National Institute of Drug Abuse (NIDA)

is sponsoring work to develop an erodible,

long-acting delivery system for the controlled administration of a narcotic antagonist to assist in the management of

narcotic addiction. Based on a favorable

relationship between potency and lack of

undesirable side effects, naltrexone has

been recommended as the drug of choice. The

requirements of the system are:

(1) Ability to release naltrexone

at prescribed rates for one

to six months.

(2) A shape and size which will

permit convenient parenteral

administration.

(3) Absence of significant adverse tissue response.

(4) Bioerodibility of the drugdelivering element.

ALZA has undertaken the task of developing a

system which is aimed at meeting these

requirements and which is based on a dispersion of naltrexone in one of ALZA's

bioerodible polymeric matrices.

OBJECTIVE

The objective of this program is to prepare

a bioerodible naltrexone delivery system

which can be implanted subcutaneously in

humans and which can release the narcotic

antagonist over one to six months at relatively constant rates sufficient to block the

euphoric action of morphine-based drugs.

CHRONOMERTM MATRICES

These novel and proprietary polymeric systems,

identified by the trademark CHRONOMERTM, are

being developed by ALZA Research, a division

of ALZA Corporation, for use in the fabrication of drug delivery systems and are

designed to undergo predictable erosion when

placed in a biological environment. In

general, the CHRONOMERTM matrix materials are

hydrophobic and absorb little water. Within

a given family of CHRONOMERTM matrix materials, variations of molecular structure make

possible the preparation of polymers which

possess a range of physical properties in39
cluding tough, glassy solids (C 101) and soft,

compliant materials (C 101 ct).

The erosion process proceeds via hydrolysis

at the surface of the CHRONOMERTM matrix.

The drug contained within that surface layer

is released into the surrounding tissues at

a predetermined rate.

the CHRONOMERTM

The erosion process of

matrices can be influenced by

incorporation of additives, particularly

alkaline materials, which modify the rate of

hydrolysis of the polymers. If a drug is

uniformly dispersed within a CHRONOMERTM

matrix, the drug is released at a rate determined by (1) its concentration in the

polymer, (2) the surface area exposed to

water, and (3) the pH of the microenvironment

at the interface between the matrix and its

surroundings.

The drug release rate at any time is proportional to the surface area of the matrix.

With some system configurations, such as thin

discs, the surface area remains essentially

constant throughout the system lifetime and

thus results in a constant drug delivery

rate. With other shapes, such as rods or

spheres, the surface area decreases with

time, causing a decrease in drug delivery

rate.

The erosion rate of CHRONOMER" materials is

inversely related to the pH of the aqueous

phase in contact with the surface of the

polymer. If the drug is acidic, its release

can increase the acidity of the environment

and hence can accelerate CHRONOMERTMerosion

and concomitant drug release. When the drug

is basic, the local hydrogen ion concentration is reduced and the CHRONOMERTM matrix

erodes at a slower rate than predicted for

the pH of the bulk of the surrounding medium.

Since relatively slow release rates are required for a naltrexone delivery system,

Naltrexone free base, rather than naltrexone

hydrochloride, is being used in CHRONOMERTM/

naltrexone composites, If necessary, release

rates can be modified by incorporating bases

or buffers into the matrix.

Prior to the commencement of the present NIDA

sponsored contract, CHRONOMERTM drug delivery

systems had been demonstrated to provide continuous, and substantially constant, release

in vitro and in vivo. However, in some cases,

there was a lag time of up to one week before

the onset of the predicted rate of erosion.

CHRONOMERTM matrix materials are presently

made in 150 g batches in a modified Atlantic

Research 2CV reactor. Our ability to control

and prescribe reaction conditions leading to

reproducible batches of polymer has improved

considerably during the last few years due to

our independent effort to perfect these unique

erodible matrix materials. For example,

neither the molecular weight, which approaches

100,000, nor the molecular weight distribution

(Mw/Mn = 3.5) of these polymers has varied by

greater than 10% in the last five consecutive

runs. A larger Atlantic Research 1OCV has

been purchased by the Corporation and could

increase our batch capacity by 200 fold. The

unit has just been received, is not installed,

and is currently undergoing a thorough inspection to evaluate compliance with

specifications.

Toxicological studies conducted at ALZA during

the past two years have not revealed limiting

adverse effects of either the CHRONOMERTM

materials or their hydrolysis products.

ANIMAL MODELS

The development of a long-acting, parenteral

delivery system for the controlled administration of naltrexone requires a demonstration of efficacy in an experimental animal.

The tail-flick test was recommended by NIDA

as the means of evaluating the pharmacological activity of naltrexone. In this test,

morphine delays the tail-flick response to

externally applied heat and the narcotic

antagonist reduces or eliminates this delay.

Either the rat or the mouse can be used in

this test; we selected the rat because it is

the more suitable for drug release kinetic

studies in vivo.

The ability to perform satisfactorily the

tail-flick test procedure was demonstrated.

The test was used to measure the effectiveness

of naltrexone to antagonize the analgesic

action of an 80% effective dose (ED80) of

morphine (found to be 6.5 mg/kg body weight)

in the rat. Single doses of naltrexone

ranging between 2 and 20 μg/kg, injected

subcutaneously in rats, were used to establish

a single dose-response curve, A linear

relationship resulted when dose (log scale)

was plotted against response (probit scale).

An 88% antagonist effect was achieved with a

naltrexone dose of 20 μg/kg. An order-ofmagnitude estimate was made to translate these

data into a continuous drug delivery rateresponse relationship. About 5 μg of naltrexone/kg/hr was calculated to be necessary

to maintain > 80% antagonism.

Experiments to determine the parenteral

dosage of continuously administered naltrexone

required to antagonize the analgesic action

of an 80% effective dose (ED80) of morphine in

the rat were recently completed. Naltrexone

was infused intravenously at doses of 0, 4

and 16 μg/kg/hr. The analgesic effect of an

40
expected ED80 dose of morphine was determined

at the end of six hours, by means of the tailflick procedure.

Four and 16 μg/kg/hr naltrexone resulted in

54 and 89% antagonism, respectively, against

a 63.5% effective dose of morphine. A more

effective morphine challenge (EDBO) is expected to lower the level of antagonism using

these rates of naltrexone delivery.

DRUG RELEASE KINETICS

In order to (1) determine the naltrexone release kinetics and shape stability of

CHRONOMERTM/naltrexone composites in vitro

and in vivo and (2) evaluate them with respect

to the degree of tissue response, naltrexone

was incorporated into C 101 and C 101 ct

matrix materials at a level of 20 mg/g. Pure

matrix materials and those filled with 10%

Na2C03 were used to afford a range of useful

lifetimes and physical characteristics. The

mixtures were fabricated into 1.3 mm thick

films and cut into 1 cm diameter circles.

The systems were chemically analyzed for naltrexone after fabrication, and the drug was

shown to be unaffected by either the processing conditions or the CHRONOMERTM

matrices.

The systems were implanted subcutaneously in

rats. Naltrexone release from the systems

was measured by analyzing the recovered systems for drug. Naltrexone release from

samples of common origin was also measured

in vitro (phosphate buffer), The results of

those experiments are shown later in the text.

All drug release studies showed a high level

of scatter. Average values of naltrexone

released in vitro from each of these systems

resulted in a release rate of 1.6 to 2.6 μg/

hr for the first week, Systems containing

Na2CO3 continued to release naltrexone at this

rate for about the next 21⁄2 weeks. Systems

without Na2CO3 released naltrexone at a

constant rate of 6.0 to 7.8 μg/hr after the

first week of the study.

stability able able

C 101, C 1Ol ct,

System C 101 N a2 C O3

C 101 ct

Shape accept- good poor acceptNa2 CO3

Rate of

drug

release

(μg/hr)

in vitro

1st week

3rd week

Tissue

response*

2nd week

4th week

2.0 1.6 1.5 1.7

7.8 1.6 6.0 2.6

6 1 0 1

0 0 1 1

*Number of moderate or severe reactions

observed.

Data obtained by measuring the naltrexone in

subcutaneously implanted devices, recovered

two and four weeks post-implantation, showed

greater scatter than that obtained in vitro.

The C 101 ct systems without alkaline excipients exhibited poor shape stability. Both

Na2CO3-free CHRONOMERTM matrices eroded within a few weeks time. Both CHRONOMERTM

matrices containing 10% Na2C03 had acceptable

erosion characteristics, although they increased in size and weight after implantation,

probably because of water absorption by the

excipient. The four CHRONOMER systems

produced some moderate tissue reaction at two

weeks post-implantation. Less tissue reaction

was observed at four weeks post-implantation,

when the systems were significantly smaller

and smoother compared with the original implant. With regard to drug release kinetics,

shape stability, tissue response and longevity, the candidate based on a C 101 matrix

containing 10% Na2CO3 was favored.

Because of the scatter observed in the drug

release data, a number of experiments were

initiated to better define the homogenity of

the CHRONOMERTM/drug mix and the drug release

kinetics in vitro. A variety of matrix/drug

formulations were prepared. Part of the

source of scatter was traced to inadequate

mixing. The mixing problem has since been

obviated by using an inexpensive but more

efficient blender, designed and developed by

ALZA. The other source of scatter was found

to result from instability of naltrexone in

the erosion medium. Relatively small amounts

41
degradation products (< 3%) result in a large

increase in the extinction coefficient of the

ultraviolet absorption spectrum of naltrexone

(as much as 40%). An improved assay procedure (liquid chromatography) is now routinely

used and this technique provides measurements

with less than a few percent error.

Using the improved techniques described above,

two matrix materials (C 101 and C 101 ct)

containing naltrexone and excipient were reevaluated. Of the formulations investigated,

CHRONOMERTM 101 ct/naltrexone systems now

appear to be the most promising. After an

initial lag interval of about two weeks,

during which little drug is released, the

C 101 ct matrix begins eroding and releases

drug in a zero order fashion at a rate of

30 - 45 μg/hr/cm2. That these C 101 ct/

naltrexone systems erode and release drug

concomitantly is evidenced by complete matrix

erosion at the time drug has disappeared from

the system. The concentration of suspended

drug in the polymer does not appear to affect

the rate of drug release at least to a concentration of 30%.

In contrast to the results with C 101 ct,

C 101 failed to provide satisfactory functionality. The lag period preceeding drug

release for CHRONOMERTM 1O1/naltrexone systems increased with decreasing drug content

up to as long as 18 days at a loading of 2%

(w/w). All C lOl/naltrexone systems had

substantial (50 - 80%) polymer residues remaining after complete drug release. We

conclude that C lOl/naltrexone systems do

not erode and release drug concomitantly,

and hence no further efforts will be expended

on C 101 systems.

Preliminary sterilization studies showed that

no adverse effects to CHRONOMERTM/naltrexone

systems occurred after exposure to 2.5 or

5.0 mrads of 60Co irradiation.

FUTURE PLANS

The proposed program utilizes the background

and experience gained in the earlier studies.

The plans are directed to the development of

the C 101 ct matrix system containing naltrexone and excipient. Our interpretation

of (1) the long lag period between exposure

to moisture and establishment of steady state

drug release from these systems and (2) the

large quantity of crystalline residue of

C 101 matrix which remains after all the drug

has been released, is based on phenomena

brought about by dissolved naltrexone in the

matrix, Basic materials are known to inhibit

the ability of these matrices to erode and

plasticizers are known to facilitate crystallization, Naltrexone is believed to play

both roles. If a less soluble (in the matrix)

form of the drug is to be used, the problem of

delayed erosion may be significantly reduced.

By shifting to C 101 ct, crystallization phenomena may be avoided completely.

42
In all three phases of this evaluation research, a substantial effort has been devoted

to the development of the techniques and

expertise necessary to carry out the evaluation in each phase. Progress in this developTESTING OF DRUG mental work has been such that all three

phases of evaluation can now be carried out

DELIVERY SYSTEMS FOR for naltrexone delivery systems.

USE IN THE TREATMENT PHARMACOLOGIC CHARACTERIZATION OF PROLONGEDANTAGONIST RELEASE

OF NARCOTIC ADDlCTlON

Richard H. Reuning, Ph.D.

College of Pharmacy, Ohio State Univ.

Louis Malspeis Ph.D.

College of Pharmacy. Ohio State Univ.

Sylan Frank, Ph.D.

College of Pharmacv. Ohio State Univ.

Robert E. Notari, Ph.D.

College of Pharmacy, Ohio State Univ.

The research on pharmacologic characterization

of prolonged antagonist release has been concerned with both the development of methodology and the actual evaluation of several

naltrexone delivery systems. In this work, a

modified version of the mouse tail-flick test

for narcotic antagonist activity that was

developed permits a reproducible temperature

to be applied to a small portion of the mouse

tail and yields data with comparatively small

standard deviations. The technique has been

critically tested and found to be an accurate

measure of analgesia and analgesic antagonism.

INTRODUCTION

Several naltrexone delivery systems have been

evaluated for duration of analgesic, antagonism

at a dose level of 40 mg naltrexone/kg using

this modified mouse tail-flick procedure. A

delivery system consisting of 35% naltrexone

in 95% polylactic acid. 5% Citroflex 4 yielded

The objective of this research is to evaluate a duration of analgesic antagonism of approxithe drug release characteristics of drug mately 17 days, compared to 1/2 day for the same

delivery systems developed for narcotic antag- dose of naltrexone in aqueous solution.

onists, particularly naltrexone and naloxone. Results from additional controls indicate that

The procedure which has been developed to the prolongation of activity was due to slow

accomplish this objective is a series of three release of naltrexone from the polymer delivsuccessive testing procedures, each more quan- ery system. A second delivery system consisttitative than the previous in describing the ing of naltrexone zinc tannate complex susrelease rate. The phase I evaluation consists pended in a vehicle of 2% aluminum monostearof a test for the duration of analgesic antag- ate in peanut oil yielded significant analonism in mice using the mouse tail-flick test. gesic antagonism for 16 days whereas naltrexPhase II of our evaluation scheme is concerned one hydrochloride suspended in the same

with quantitating the excretion of tritium in vehicle lasted only 4 days. Results from

urine and feces after administration of a drug additional controls indicate that the mechandelivery system containing radiolabeled narco- ism responsible for the prolonged activity is

tic antagonist to rats or guinea pigs. The slow dissolution of naltrexone from the delivthird phase of this evaluation work is a phar- ery system. A somewhat similar delivery sysmacokinetic description of the release rate of tem, i.e., naltrexone aluminum tannate suspendnarcotic antagonist from the delivery systems ed in a vehicle of 2% aluminum monostearate in

administered to dogs and monkeys. In this peanut oil, yielded significant analgesic

phase III work the narcotic antagonist is antagonism for 22 days. The fourth delivery

administered in unlabeled form and samples of system consisted of beads containing 50% nalblood and/or urine are subsequently collected trexone in a co-polymer of L(+)-lactic acid

and analyzed for concentration of the narcotic (75%)/glycolic acid (25%). This system

antagonist by electron capture gas-liquid yielded significant antagonism for 18 days at

chromatography. Such concentration data a 50 mg/kg dose level of naltrexone and for at

obtained as a function of time, together with least 21 days at a 100 mg/kg dosage. Again,

appropriate control experiments, permit a comparison with appropriate controls indicates

determination of the release rate of the nar- that the prolonged analgesic antagonism is due

cotic antagonist from the drug delivery sys- to slow release of naltrexone from the delivtem. ery system.

43
SEMI-QUANTITATIVE CHARACTERIZATION OF PROLONGED ANTAGONIST RELEASE

In order to carry out the proposed research on

estimating narcotic antagonist release rates

from delivery systems containing tritiated

antagonist, it has been necessary to spend

considerable effort in developing methodology

prior to evaluating release of naltrexone from

a sustained-release delivery system. Initially, both the radiochemical purity of tritiated

naltrexone and the lack of exchange of label

with water in biological systems was established for both 8-3

H-naltrexone and 15,16-3H

naltrexone. Stability of the tritiated naltrexone was monitored periodically by thinlayer chromatography. Essentially quantitative recovery of the administered tritiated

naltrexone was obtained in urine and feces

after parenteral administration of a solution

of the drug to rats and guinea pigs. The

quantitation achieved by combustion of urine

and fecal samples followed by liquid scintillation counting indicates that, in the rat,

about 60% of the administered radioactivity is

excreted in feces and 40% in urine. On the

other hand, in the guinea pig only 14% of the

administered radioactivity was found in feces

and about 84% in urine.

The naltrexone zinc tannate delivery system

mentioned previously was evaluated by this

radioactivity excretion technique. 3

H-15,16-

naltrexone was administered via this dosage

form intramuscularly (20 mg/kg) in guinea

pigs. The overall elimination of radioactivity was found to be approximately exponential

in four guinea pigs with 50% of the dose excreted in 2.7-4.9 days and 90% in 11.6-15.3

days. This compares favorably with the 16

day duration of analgesic antagonism obtained

for this delivery system by the pharmacologic

technique discussed in the previous section.

PHARMACOKINETIC EVALUATION OF PROLONGED

ANTAGONIST RELEASE IN DOG,MONKEY AND MAN

Analytical Development

A major part of the work aimed at an eventual

pharmacokinetic quantitation of narcotic

antagonist release rates has been devoted to

the development of sensitive gas chromatographic-electron capture (GLC/EC) assays for

naltrexone and naloxone in human, dog and

monkey plasma. These assays for the

unchanged narcotic antagonist, which can

readily be adapted to assays for drug in

urine as well, permit a pharmacokinetic quantitation of the disposition of naltrexone and

naloxone in the dog, monkey or man, as well

as a quantitation of the release rate of

these compounds from antagonist delivery

systems. The assays for naltrexone and naloxone generally involve addition of an internal standard to the plasma sample, extraction

of the basified plasma sample with benzene,

back extraction into 0.1 N sulfuric acid,

basification, re-extraction into benzene,

evaporation, derivatization in benzene with

pentafluoropropionic anhydride or heptafluorobutyric anhydride in benzene, washing the

reaction mixture with sodium tetraborate, and

injecting a portion of the derivatized material onto an OV-17 column. The assays for

naloxone and naltrexone appear to be specific

for unchanged drug and to clearly separate the

unchanged drug from known metabolites. The

sensitivity limit is 5-10 ng of antagonist in

a 0.5 ml sample of plasma. We have extended

the sensitivity of the assay of naltrexone in

dog and monkey plasma to 1 ng/ml of plasma by

the use of 2-3 ml of plasma. This appears to

be the lower sensitivity limit of the GLC/EC

technique. Linear calibration curves have

been obtained for naloxone and naltrexone in

dog and human plasma and for naltrexone in

monkey plasma and for naltrexone in both unhydrolyzed and glusulase-hydrolyzed dog urine.

Preliminary work on a GLC/EC assay for cyclazocine indicates that it may be possible to

analyze this narcotic antagonist at concentrations less than 1 ng/ml.

Biologic Disposition of Naltrexone

In order to determine the rate of release of

narcotic antagonist from a delivery system by

pharmacokinetic techniques, it is necessary

that the assay utilized to measure the concentration of antagonist in plasma and urine be

specific for the unchanged drug. Thus, it is

essential that a knowledge of the basic metabolic pathways of the narcotic antagonist be

developed and that samples of authentic metabolites be synthesized or isolated and tested

for interference with the assay procedure.

In addition, a knowledge of the physiologic

distribution of the narcotic antagonist can

aid in explaining the relationship between the

concentration of antagonist measured in the

plasma and the amount of drug remaining in

other parts of the body. Thus far, our work

on biologic disposition has been limited to

naltrexone.

Our research on the physiologic distribution

of naltrexone has included a determination of

the extent of binding of the drug in plasma

from several species. It was found by equilibrium dialysis that naltrexone is bound in

plasma to an extent of 2O-26% in man, monkey,

dog, guinea pig, rat and mouse. The extent

of binding was independent of naltrexone concentration over a wide concentration range

in all species. A second approach to the

investigation of naltrexone distribution

involved a determination of the

44
tissue levels of radioactivity in mice at 1,

5, and 15 minutes after intravenous administration of tritiated naltrexone. The results

of this study indicate that the radioactivity

is very rapidly distributed from plasma to

tissues, concentrating in the liver, kidney

and gastrointestinal tract, with less than 4%

of the dose being present in plasma 1 minute

after administration.

Considerable progress has also been made concerning the metabolism of naltrexone in several species. Two metabolites of naltrexone,

and naltrexol, as well as two metabolites

of naloxone, and naloxol, have been synthesized by sodium borohydride reduction of

naltrexone and naloxone, respectively. Each

pair of epimers was separated by preparative

thin-layer chromatography and the structures

were determined by spectral techniques. Use

of these authentic metabolites has permitted

the development of a qualitative GLC/EC method

for distinguishing either o r naltrexol

in urine in the presence of large amounts of

naltrexone . After parenteral administration

of naltrexone substantial quantities of

naltrexol and/or its conjugates were found in

enzymatically hydrolyzed urine samples from

man, monkey and guinea pig whereas smaller

quantities were excreted by the rabbit, mouse,

rat and dog. Trace amounts of the 6 -hydroxy

epimer, a-naltrexol, were detected in the

urine of monkeys and guinea pigs only. In

vitro experiments with guinea pig liver indicate that the enzymes responsible for the reduction of naltrexone to both and

naltrexol are present in the 100,000 x g

supernatant of the liver homogenate.

The isolation of and naltrexol has also

permitted the development of a GLC/EC analytical procedure for each of these metabolites.

Linear calibration curves were obtained after

extraction of these metabolites from dog

plasma containing 20 to 150 ng of either

o r naltrexol. As mentioned previously, we

have also confirmed that these metabolites do

not interfere with the GLC/EC assay for naltrexone.

Pharmacokinetics of Naltrexone

The characterization of the phannacokinetics

of a narcotic antagonist is a necessary prerequisite to the determination of the rate of

release of that antagonist from a delivery

system. Since the species most likely to be

useful for testing narcotic antagonist delivery systems are the dog and monkey, the

pharmacokinetics of naltrexone has been

studied in these species.

Plasma concentration-time data for intravenous naltrexone in the dog, obtained by the

GLC/EC analytical technique, indicate that at

least two “compartments” are necessary to

account for the data. The half-life of the

principal exponential phase of the plasma

level-time curve was 32 and 36 min in two foxhounds, and the apparent volume of distribution was 3.2 and 3.6 liters/kg. In similar

i.v. bolus experiments using four monkeys,

the plasma level-time half-life for naltrexone

was 40-82 minutes (mean, 59 min) and the

apparent volume of distribution was 4.4-10.1

liters/kg (mean, 6.4 liters/kg). These data

indicate that naltrexone is distributed from

blood to the rest of the body to an unusually

great extent. Also, the drug appears to be

rapidly eliminated in both the dog and the

monkey. Since the clearance of naltrexone is

greater than liver blood flow, the data indicate that some extrahepatic elimination occurs

with this drug. Most importantly, however,

the phannacokinetic data from these experiments will permit a determination of the

release rate of naltrexone from a naltrexone

delivery system administered to the dog or

monkey in a subsequent experiment. Such an

experiment in the monkey is currently in

progress.

GPO 895-817

45
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