Phospholipid delivery vehicle for aqueous-insoluble active ingredients

Abstract: Compositions comprising phospholipid-encapsulated vesicles of active ingredient and triglyceride are described. In a preferred embodiment the vesicles are composed of a hexa­ methylmelamine active ingredient, with trilaurin or trimyristin as the triglyceride and a mixture of distearoylphosphatidyl­ choline, distearoylphosphatidylglycerol and cholesterol in the phospholipid outer layer. Preferably the molar ratios of active ingredient: triglyceride : DSPC : CHOL : DSPG will be from about 1:4:2:1:0 to about 1:4:1:1:1. Glycerol may be added to the carrier phase to reduce agglomeration. The composition may be used to deliver otherwise aqueous-insoluble agents to humans or animals to treat, for example, tumors.

Description: FIELD OF INVENTION This invention relates to phospholipid-encapsulated medicinal agents. It is directed in one aspect to phospholipid-encapsulated hexamethylmelamine. In another aspect it relates to the use of such compositions to deliver medicinal agents to the body, as for example to tumor cells. BACKGROUND Although a significant number of substances are known to have antitumor activity, problems have persisted in many cases in developing compositions and methods for safely and effectively delivering such substances to tumor cells. The general toxicity of many anticancer agents prevents their being administered in free form in the body. Many anticancer agents are not sufficiently soluble or stable in the aqueous environment to allow injection or other effective administration. Furthermore, it is frequently useful to control the size of delivery agents in order to achieve targeting to tumor cells or to allow filtration for the purpose of removing deleterious components such as bacteria. It is also important to achieve a composition which, apart from being non-toxic, is biocompatible. Phospholipid-encapsulated delivery vehicles have been used to overcome such problems in certain cases. It is known, for example, that some aqueous-insoluble drugs can be incorporated into the lipophilic region within the phospholipid bilayer of a liposome to achieve an aqueous-soluble, relatively non-toxic and biocompatible delivery vehicle. Not all aqueous-insoluble materials are susceptible to such a composition, however. GB-A-2018712 describes xenobiotic delivery vehicles which are microreservoirs comprising an inner core of a drug such as adriamycin or imidocarb, and a cholesterol ester, such as cholesterol oleate, or triglyceride, such as glycerol trioleate, and one or more outer layers of phospholipid material. US-A-4610868 describes lipid matrix carriers of 500 to 100 000 nm in diameter, which are produced by forming an emulsion of water, a pharmaceutical, and hydrophobic and amphiphatic lipid components, and extruding this emulsion through an orifice into an organic solvent such as acetone. Hexamethylmelamine (HXM) is an example of an anticancer agent which has received only limited use due to its poor aqueous solubility. Oral administration of HXM yields variable absorption and erratic drug concentrations in the plasma. Ames et al., Cancer Treatment Reports , Vol. 66, No. 7, pp. 1579-1581 (July 1982). Gentisate and hydrochloride salts of HXM have resulted in severe local irritation upon intravenous administration to humans. Recent attempts to formulate HXM in an intravenously-acceptable preparation have focused on incorporating the drug into fat emulsions, and have achieved HXM concentrations of 2 mg/ml or more. Intraperitoneal formulations have also focused on fat emulsions such as that formed with the oil emulsion vehicle Intralipid (Cutter Laboratories, Berkeley, California), discussed by Wickes et al., in Cancer Treatment Reports , Volume 69, No. 6, pp. 657-662 (June 1985). Although such formulations succeed in increasing the concentration of HXM to levels suitable for affecting tumor cells, they do not address the problem of targeting tumor cells specifically through use of phospholipid-encapsulated vesicles of an appropriate size. Nor do they address the problem of sterilization where the medicinal or other component may not be heat-stable since such a preparation can not be sterile filtered. Accordingly, it is an object of the present invention to provide new compositions for the formulation and delivery of aqueous-insoluble medicinal agents to the body. In one aspect, the invention provides compositions for the formulation and delivery of anticancer agents, including hexamethylmelamine. It is another object of the present invention to provide methods for manufacturing, sterilization and use of such compositions to deliver medicinal agents to the body, and in particular to tumor cells. The present invention involves compositions suitable for the delivery of a substantially aqueous-insoluble active ingredient to humans or animals comprising vesicles in a pharmaceutically acceptable carrier, wherein said vesicles are of a size of 30 to 200 nanometers in diameter and comprise (1) a substantially aqueous-insoluble active ingredient in mixture with trilauroylglycerol and (2) an encapsulating layer including a phospholipid material. It is thought that the emulsified vesicles have a roughly spherical outer monolayer of phospholipids with hydrophobic tails of the phospholipid molecules oriented inwardly toward the medicinal active ingredient/lipid triglyceride phase. A preferred active ingredient is the anticancer agent hexamethylmelamine. The phospholipid outer coating preferably comprises one or more phospholipid materials having from 12 to 20 carbons in the alkyl chains. Distearoylphosphatidylcholine and distearoylphosphatidylglycerol are preferred in the case of the active ingredient hexamethylmelamine. Cholesterol may also be added to the compositions. Preparation of the compositions may be carried out using standard procedures in an appropriate saline or saccharide-based carrier solution. Glycerol may also be added to the aqueous carrier to minimize aggregation of the final compositions. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a cross-sectional schematic illustration of the theoretical structure of the delivery vehicle of the present invention. DETAILED DESCRIPTION OF THE INVENTION As indicated above, the present invention involves the encapsulation and improved delivery of aqueous-insoluble active ingredients, and in particular insoluble anticancer agents such as hexamethylmelamine, in phospholipid vesicles. The present compositions may be used in some cases where other delivery vehicles, such as liposomes, are not satisfactory. Hexamethylmelamine (HXM), or 2,4,6-Tris(dimethylamino)-s-triazine, is an anticancer compound that is very similar structurally to the alkylating agent triethylenemelamine. Its structure is as follows: As discussed above, the poor aqueous solubility of HXM has hindered its usefulness in anticancer therapy. Investigations relating to the present invention have shown that improved solubilization of HXM in a phospholipid vesicle may be achieved with the present delivery vehicles as compared to liposomal compositions. The present compositions are thought to have a structure as shown in Fig. 1 in cross-section. The delivery vehicle may be roughly spherical in shape. The inner phase of the delivery vehicle includes the active ingredient dissolved in the lipid trilauroylglycerol (trilaurin). Because this inner phase is essentially lipophilic, it will form a stable association with an encapsulating monolayer of phospholipids. The hydrophilic nature of the outer surface of the encapsulating layer allows aqueous and in vivo solubilization, and may achieve other advantages associated with liposomal structures (including biocompatibility, isolation of active ingredient toxicity and targeting of tumor cells). It is necessary to utilize an appropriate lipid triglyceride in order to achieve a satisfactory delivery vehicle. A given active ingredient may be soluble in a number of triglycerides, or it may be made soluble by, for example, altering pH or ionic strength of the mixture or by complexing the active ingredient with a second lipid-soluble agent. Nevertheless, not all lipid triglycerides that can solubilize a given active ingredient will necessarily be compatible with a stable phospholipid emulsion. For example, fully-saturated long chain triglycerides such as tripalmitin and tristearin solubilize HXM upon heating but tend to form a hard waxy composition upon cooling that cannot be satisfactorily emulsified with the phospholipids tested. Conversely, the long unsaturated alkyl chains in the triglyceride triolein will allow an emulsion with phospholipids, but are susceptible to oxidation. Shorter-chain triglycerides such as triacetin and tributyrin poorly solubilize HXM and do not form stable emulsions in the compositions tested. In the case of, for example, the active ingredient HXM, the triglyceride trilaurin is preferred. This triglyceride will solubilize the active ingredient and is able to form stable emulsions. The active ingredient to be used herein will typically be one or more compounds which are insoluble in aqueous media or which require enhanced solubilization to achieve useful concentration. With respect to HXM, for example, solubility of the free drug in saline solution has been reported to be as low as 0.070 mg/ml (Wickes et al., Cancer Treatment Reports , Volume 69, No. 6, pp. 657-662 (June 1985)) and as high as 0.20 mg/ml in water (Ames et al., Cancer Treatment Reports , Volume 66, No. 7, pp. 1579-1581 (July 1982)). A desirable concentration would exceed 1.0 mg/ml measured with respect to HXM content. Although the active ingredients useful in the present invention will typically be agents that are difficult to solubilize in aqueous media, this need not necessarily be the case. So long as the active ingredient can be made compatible with the triglyceride phase, and so long as it is desirable to encapsulate the ingredient in a phospholipid monolayer, the present invention may yield a useful delivery vehicle. As discussed above, modifications to the pH or ionic strength of the mixture, or modifications to the active ingredient such as complexation, may be employed to render the ingredient triglyceride-soluble. Similar modifications may be made to allow formation of a stable phospholipid vesicle. The outer phospholipid coating may be composed of a range of phospholipids including neutral phospholipids such as phosphatidylcholines and phosphatidylethanolamines, as well as ionic phospholipids such as phosphatidylglycerols and phosphatidylserines. Preferred phospholipids are those having from 12 to 20 carbons in their alkyl side chains. Cholesterol may also be added as component of the outer layer and is preferred in many cases. Distearoylphosphatidylcholine is a particularly preferred phospholipid with the active ingredient HXM and the inner phase triglyceride trilaurin. The anion distearoylphosphatidylglycerol may also be added to yield a successful composition. Cholesterol is a preferred component in the outer coating. The molar ratio of ingredients in such a HXM composition will preferably range as follows: HXM: 1 Distearoylphosphatidylcholine: 2-1 Cholesterol: 1 Distearoylphosphatidylglycerol: 0-1 Trilaurin: 4 The formation of the emulsified delivery vehicles may be achieved in a saline solution, as for example a 0.9% solution of sodium choloride in water, or in a saccharide or disaccharide solution, such as 5% dextrose or 9% lactose in water. In addition, it is often preferred to add glycerol to the mixture in a concentration of about 100 mM in order to reduce or eliminate any adverse tendency toward aggregation of the vesicles. Formation of the vesicles may be achieved using standard sonication techniques. Such is the case with the HXM compositions described herein. Non-solubilized material may be removed from the mixture by centrifugation. Further purification may include filtration, for example, through a 5-micrometer filter needle to ascertain syringeability, and through a 0.45 and/or 0.22-micrometer filter to remove, e.g., bacterial contaminants. The final delivery vehicle vesicles will preferably be smaller than 100 nm in diameter, and preferably in the range 40-75 nm in diameter. The following examples demonstrate the preparation and characterization of one form of the delivery vehicles of the present invention. Example 2 is given to compare one of the present compositions with a liposome-type delivery vehicle tested for use with HXM. EXAMPLE 1 Preparation of a Hexamethylmelamine-Trilaurin Delivery Vehicle Encapsulated With Phospholipid. Hexamethylmelamine was supplied by the National Cancer Institute, compound NSC-13875, lot number H739646. Solubility tests were run for HXM in a number of pure trigylcerides and it was determined that HXM was highly soluble (more than 50 mg/ml) in tributyrin, trihexanoin, tricaprylin, trilaurin and tripalmitin (Sigma Chemical Co., St. Louis, MO). Emulsions of HXM were formed by heating a measured quantity of the triglyceride to a liquid state and adding with stirring measured quantities of HXM, phospholipid (Avanti Biochemicals, Birmingham, AL), cholesterol (Sigma) and, finally, the aqueous solution phase. The solution was then sonicated under an inert atmosphere using a probe type sonicator (Sonics and Materials, Model VCS-500, Danbury, CT). The sample was then centrifuged at 750 g for ten minutes and the amount of precipitate estimated. An alternate composition was sought if the total precipitation was greater than about 20% of the starting material. Preferably, the precipitate fraction would be less than 10%. Following filtration of the emulsion of delivery vehicles through a 5-micrometer filter needle to verify syringeability, the samples were filtered through 0.45 and/or 0.22 micrometer microfilters. They were then analyzed for total HXM concentration and for evidence of any HXM decomposition using thin layer chromatography on silica gel 60 plates (Merck), high pressure liquid chromatography and/or UV/visible spectroscopy using a Perkin-Elmer Lamda 3B spectrophotometer. The results of such procedures are summarized in Table 1. Table 1 demonstrates that useful concentrations of HXM in aqueous solution may be achieved using the compositions of the present invention. Preferred formulations use trilaurin in the inner phase, and 100 mM glycerol dissolved in the aqueous phase. Analysis using UV/visible spectroscopy of the four trilaurin compositions tested for final HXM concentration showed no noticeable difference from the starting drug. Thin layer chromatography was also consistent with intact HXM in these cases. A repeated UV/visible spectroscopic analysis after 24 hours indicated diminished absorbance at 227 nm, suggesting a decrease in aqueous HXM from about 3.0 to 2.2 mg/ml. The later spectra were consistent with intact HXM. An alternate procedure for formulating the present compositions, useful especially for small batches, involves dissolving each desired component in an organic solvent such as chloroform, mixing appropriate volumes of each chloroform solution, evaporating the chloroform under vacuum to obtain a lipid-drug-triglyceride film, and then adding this film to the appropriate aqueous phase as dicussed above. EXAMPLE 2 Incorporation of Hexamethylmelamine Into Liposomal Delivery Vehicles By way of comparison, attempts were made to incorporate HXM into the intra-bilayer phospholipid region of a liposome without use of any triglyceride. Appropriate proportions of HXM, phospholipid and cholesterol were dissolved in an organic solvent such as chloroform (distearoylphosphatidylglycerol was dissolved in 1:1 methanol:chloroform and then mixed with the chloroform solution). The solvent was then removed under reduced pressure to yield a lipid-drug film. This film was then mixed and sonicated as above in an appropriate aqueous solvent to yield small unilamellar liposomal vesicles. As above, the addition of 100 mM glycerol prevented agglomeration in some instances. Following centrifugation, the liposomes were filtered and analyzed for HXM concentrations in aqueous solution. Results of these tests showed that addition of the anion distearoylphosphatidylglycerol increased the amount of membrane-incorporated HXM by promoting partitioning of the drug into the lipid phase. However, the final concentration of HXM achieved did not in such cases reach the desired level of at least 1.0 mg/ml. Based on these results, the desirability of the alternative aqueous solubilization vehicles disclosed herein becomes clear.

Claims: Claims for the following Contracting States : AT, BE, CH, DE, FR, GB, IT, LU, NL, SE A composition suitable for the delivery of a substantially aqueous-insoluble active ingredient comprising vesicles in a pharmaceutically acceptable carrier, wherein said vesicles are of a size of 30 to 200 nanometers in diameter and comprise (1) a substantially aqueous-insoluble active ingredient in mixture with trilauroylglycerol and (2) an encapsulating monolayer including a phospholipid material. The composition of claim 1 wherein the encapsulating layer includes cholesterol. The composition of any one of the preceding claims wherein the phospholipid material is at least one phospholipid having an alkyl side chain of 12 to 20 carbon atoms in length. The composition of claim 3 wherein the phospholipid material includes an anionic phospholipid component. The composition of claim 1 or 2 wherein the phospholipid material comprises a mixture of dialkylphosphatidylcholine and dialkylphosphatidylglycerol compounds having alkyl side chains of 12 to 20 carbon atoms in length. The composition of claim 5 wherein the encapsulating layer includes distearoylphosphatidylcholine, distearoylphosphatidylglycerol and cholesterol. A composition according to any one of claims 1 to 6, wherein the active ingredient is an antitumour compound, for use in a method of treating neoplastic tumours in a human body by parenteral administration. A composition according to claim 7 wherein the active ingredient is hexamethylmelamine. A method of making a composition as defined in any one of the preceding claims comprising: (1) combining said active ingredient with trilauroylglycerol, a phospholipid material and a pharmaceutically acceptable carrier; (2) forming vesicles containing the active ingredient; and (3) removing undesirable materials from the resulting composition. The method of claim 9 wherein the active ingredient is hexamethylmelamine and 1 molar fraction of hexamethylmelamine is combined with about 4 molar fractions trilauroylglycerol, 2-1 molar fractions distearoylphosphatidylcholine, about 1 molar fraction cholesterol, and 0-1 molar fractions distearoylphosphatidylglycerol. A method of making a composition of vesicles suitable for the delivery of a substantially aqueous-insoluble active ingredient, wherein said vesicles are of a size of 30 to 200 nanometers in diameter, comprising forming a triglyceride solution of the aqueous-insoluble active ingredient and a phospholipid and then forming vesicles comprising an outer monolayer phospholipid coat and an enclosed phase containing the active ingredient and a triglyceride material in an aqueous, pharmaceutically acceptable carrier. Claims for the following Contracting States : ES, GR A method of making a composition suitable for the delivery of a substantially aqueous-insoluble active ingredient comprising vesicles in a pharmaceutically acceptable carrier, wherein said vesicles are of a size of 30 to 200 nanometers in diameter and comprise (1) a substantially aqueous-insoluble ingredient in mixture with trilauroylglycerol and (2) an encapsulating monolayer including a phospholipid material, said method comprising: (1) combining said active ingredient with trilauroylglycerol, a phospholipid material and a pharmaceutically acceptable carrier; (2) forming vesicles containing the active ingredient; and (3) removing undesirable materials from the resulting composition. The method of claim 1 wherein the encapsulating layer includes cholesterol. The method of any one of the preceding claims wherein the phospholipid material is at least one phospholipid having an alkyl side chain of 12 to 20 carbon atoms in length. The method of claim 3 wherein the phospholipid material includes an anionic phospholipid component. The method of claim 1 or 2 wherein the phospholipid material comprises a mixture of dialkylphosphatidylcholine and dialkylphosphatidyl-glycerol compounds having alkyl side chains of 12 to 20 carbon atoms in length. The method of claim 5 wherein the encapsulating layer includes distearoylphosphatidylcholine, distearoylphosphatidylglycerol and cholesterol. A method according to any one of claims 1 to 6 wherein the active ingredient is an antitumour compound, for use in a method of treating neoplastic tumours in a human body by parenteral administration. A method according to claim 7 wherein the active ingredient is hexamethylmelamine. The method of any one of claims 1 to 8 wherein the active ingredient is hexamethylmelamine and 1 molar fraction of hexamethylmelamine is combined with about 4 molar fractions trilauroylglycerol, 2-1 molar fractions distearoylphosphatidylcholine, about 1 molar fraction cholesterol, and 0-1 molar fractions distearoylphosphatidylglycerol. A method of making a composition of vesicles suitable for the delivery of a substantially aqueous-insoluble active ingredient, wherein said vesicles are of a size of 30 to 200 namometers in diameter, comprising forming a triglyceride solution of the aqueous-insoluble active ingredient and a phospholipid and then forming vesicles comprising an outer monolayer phospholipid coat and an enclosed phase containing the active ingredient and a triglyceride material in an aqueous, pharmaceutically acceptable carrier.

Inventors: FORSSEN ERIC ANTON, FORSSEN, ERIC ANTON
Applicants: VESTAR INC, VESTAR, INC.
Application year: 1988
Main classification: A61K9
IPC codes: A61K9, A61K31, A61K47, A61P35