Guidance for Industry 
Estimating the Maximum Safe 
Starting Dose in Initial Clinical Trials 
for Therapeutics in Adult Healthy 
Volunteers 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Drug Evaluation and Research (CDER) 
 
July 2005 
Pharmacology and Toxicology 
 
J:\!GUIDANC\5541fnlcln1.doc 
07/06/05   
Guidance for Industry 
Estimating the Maximum Safe 
Starting Dose in Initial Clinical Trials 
for Therapeutics in Adult Healthy 
Volunteers 
 
 
 
Additional copies are available from: 
 
Office of Training and Communications 
Division of Drug Information, HFD-240 
Center for Drug Evaluation and Research 
Food and Drug Administration 
5600 Fishers Lane 
Rockville, MD  20857 
(Tel) 301-827-4573 
http://www.fda.gov/cder/guidance/index.htm 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Drug Evaluation and Research (CDER) 
 
July 2005 
Pharmacology and Toxicology 
 
 
   
TABLE OF CONTENTS 
 
I.  INTRODUCTION.............................................................................................................1 
II.  BACKGROUND...............................................................................................................2 
III.  OVERVIEW OF THE ALGORITHM...........................................................................3 
IV.  STEP 1:  NO OBSERVED ADVERSE EFFECT LEVEL DETERMINATION........5 
V.  STEP 2:  HUMAN EQUIVALENT DOSE CALCULATION......................................6 
A.  Conversion Based on Body Surface Area....................................................................................6 
B.  Basis for Using mg/kg Conversions..............................................................................................7 
C.  Other Exceptions to mg/m2 Scaling Between Species.................................................................8 
VI.  STEP 3:  MOST APPROPRIATE SPECIES SELECTION.........................................9 
VII.  STEP 4:  APPLICATION OF SAFETY FACTOR.......................................................9 
A.  Increasing the Safety Factor.......................................................................................................10 
B.  Decreasing the Safety Factor......................................................................................................11 
VIII.  STEP 5:  CONSIDERATION OF THE PHARMACOLOGICALLY ACTIVE 
DOSE................................................................................................................................12 
IX.  SUMMARY.....................................................................................................................12 
REFERENCES............................................................................................................................13 
GLOSSARY.................................................................................................................................15 
APPENDIX A:.............................................................................................................................16 
Analysis of Allometric Exponent on HED Calculations....................................................................16 
APPENDIX B:.............................................................................................................................18 
Analysis of Body Weight Effects on HED Calculations....................................................................18 
APPENDIX C:.............................................................................................................................24 
Derivation of the Interspecies Scaling Factor (W /W )(1-b)................................................................24 
a h
APPENDIX D:.............................................................................................................................25 
Examples of Calculations for Converting Animal Doses to Human Equivalent Doses..................25 
APPENDIX E:.............................................................................................................................27 
Selection of Maximum Recommended Starting Dose for Drugs Administered Systemically to 
Normal Volunteers...............................................................................................................................27 
 
  Contains Nonbinding Recommendations 
 
1
Guidance for Industry  
Estimating the Maximum Safe Starting Dose in Initial Clinical 
Trials for Therapeutics in Adult Healthy Volunteers 
 
 
 
 
This guidance represents the Food and Drug Administrations (FDAs) current thinking on this topic.  It 
does not create or confer any rights for or on any person and does not operate to bind FDA or the public.  
You can use an alternative approach if the approach satisfies the requirements of the applicable statutes 
and regulations.  If you want to discuss an alternative approach, contact the FDA staff responsible for 
implementing this guidance.  If you cannot identify the appropriate FDA staff, call the appropriate 
number listed on the title page of this guidance.  
 
 
 
 
I.  INTRODUCTION 
 
This guidance outlines a process (algorithm) and vocabulary for deriving the maximum 
recommended starting dose (MRSD) for first-in-human clinical trials of new molecular entities 
in adult healthy volunteers, and recommends a standardized process by which the MRSD can be 
selected.  The purpose of this process is to ensure the safety of the human volunteers.  
 
The goals of this guidance are to:  (1) establish a consistent terminology for discussing the 
starting dose; (2) provide common conversion factors for deriving a human equivalent dose 
(HED); and (3) delineate a strategy for selecting the MRSD for adult healthy volunteers, 
regardless of the projected clinical use.  This process is depicted in a flow chart that presents the 
decisions and calculations used to generate the MRSD from animal data (see Appendix E).   
 
FDAs guidance documents, including this guidance, do not establish legally enforceable 
responsibilities.  Instead, guidances describe the Agencys current thinking on a topic and should 
be viewed only as recommendations, unless specific regulatory or statutory requirements are 
cited.  The use of the word should in Agency guidances means that something is suggested or 
recommended, but not required. 
 
 
                                                 
1 This guidance has been prepared by the Office of New Drugs in the Center for Drug Evaluation and Research 
(CDER) at the Food and Drug Administration.  
  1 Contains Nonbinding Recommendations 
 
II.  BACKGROUND 
 
The process identified in this guidance pertains to determining the MRSD for adult healthy 
subjects when beginning a clinical investigation of any new drug or biological therapeutic that 
has been studied in animals.  This guidance is not pertinent to endogenous hormones and 
proteins (e.g., recombinant clotting factors) used at physiologic concentrations or prophylactic 
vaccines.  The process outlined in this guidance pertains primarily to drug products for which 
systemic exposure is intended; it does not address dose escalation or maximum allowable doses 
in clinical trials. 
 
Although the process outlined in this guidance uses administered doses, observed toxicities, and 
an algorithmic approach to calculate the MRSD, an alternative approach could be proposed that 
places primary emphasis on animal pharmacokinetics and modeling rather than dose (Mahmood 
et al. 2003; Reigner and Blesch 2002).  In a limited number of cases, animal pharmacokinetic 
data can be useful in determining initial clinical doses.2  However, in the majority of 
investigational new drug applications (INDs), animal data are not available in sufficient detail to 
construct a scientifically valid, pharmacokinetic model whose aim is to accurately project an 
MRSD. 
 
Toxicity should be avoided at the initial clinical dose.  However, doses should be chosen that 
allow reasonably rapid attainment of the phase 1 trial objectives (e.g., assessment of the 
therapeutics tolerability, pharmacodynamic or pharmacokinetic profile).  All of the relevant 
preclinical data, including information on the pharmacologically active dose, the full toxicologic 
profile of the compound, and the pharmacokinetics (absorption, distribution, metabolism, and 
excretion) of the therapeutic, should be considered when determining the MRSD.  Starting with 
doses lower than the MRSD is always an option and can be particularly appropriate to meet some 
clinical trial objectives. 
                                                 
2 If the parent drug is measured in the plasma at multiple times and is within the range of toxic exposures for two or 
more animal species, it may be possible to develop a pharmacokinetic model predicting human doses and 
concentrations and to draw inferences about safe human plasma levels in the absence of prior human data.  Although 
quantitative modeling for this purpose may be straightforward, the following points suggest this approach can 
present a number of difficulties when estimating a safe starting dose.  Generally, at the time of IND initiation, there 
are a number of unknowns regarding animal toxicity and comparability of human and animal pharmacokinetics and 
metabolism:  (1) human bioavailability and metabolism may differ significantly from that of animals; (2) 
mechanisms of toxicity may not be known (e.g., toxic accumulation in a peripheral compartment); and/or (3) 
toxicity may be due to an unidentified metabolite, not the parent drug.  Therefore, relying on pharmacokinetic 
models (based on the parent drug in plasma) to gauge starting doses would require multiple untested assumptions.  
Modeling can be used with greatest validity to estimate human starting doses in special cases where few underlying 
assumptions would be necessary.  Such cases are exemplified by large molecular weight proteins (e.g., humanized 
monoclonal antibodies) that are intravenously administered, are removed from circulation by endocytosis rather than 
metabolism, have immediate and detectable effects on blood cells, and have a volume of distribution limited to the 
plasma volume.  In these cases, allometric, pharmacokinetic, and pharmacodynamic models have been useful in 
identifying the human mg/kg dose that would be predicted to correlate with safe drug plasma levels in nonhuman 
primates.  Even in these cases, uncertainties (such as differences between human and animal receptor sensitivity or 
density) have been shown to affect human pharmacologic or toxicologic outcomes, and the use of safety factors as 
described in this guidance is still warranted. 
  2 Contains Nonbinding Recommendations 
 
 
The remainder of this guidance focuses on the recommended algorithmic process for starting 
dose extrapolation from animals to humans based on administered doses, since this method will 
likely be useful for the majority of INDs seeking to investigate new drugs in healthy volunteers.  
Some classes of drugs (e.g., many cytotoxic or biological agents) are commonly introduced into 
initial clinical trials in patient volunteers rather than healthy volunteers.  Typically, patients are 
used instead of healthy volunteers when a drug is suspected or known to be unavoidably toxic.  
This guidance does not address starting doses in patients.  However, many principles and some 
approaches recommended here may be applicable to designing such trials.  
 
 
III.  OVERVIEW OF THE ALGORITHM 
 
The recommended process for selecting the MRSD is presented in Appendix E and described in 
this section.  The major elements (i.e., the determination of the no observed adverse effect levels 
(NOAELs) in the tested animal species, conversion of NOAELs to HED, selection of the most 
appropriate animal species, and application of a safety factor) are all discussed in greater detail in 
subsequent sections.  Situations are also discussed in which the algorithm should be modified.  
The algorithm is intended to be used for systemically administered therapeutics.  Topical, 
intranasal, intratissue, and compartmental administration routes and depot formulations can have 
additional considerations, but similar principles should apply.   
 
The process of calculating the MRSD should begin after the toxicity data have been analyzed.  
Although only the NOAEL should be used directly in the algorithm for calculating an MRSD, 
other data (exposure/toxicity relationships, pharmacologic data, or prior clinical experience with 
related drugs) can affect the choice of most appropriate species, scaling, and safety factors.   
 
The NOAEL for each species tested should be identified, and then converted to the HED using 
appropriate scaling factors.  For most systemically administered therapeutics, this conversion 
should be based on the normalization of doses to body surface area.  Although body surface area 
conversion is the standard way to approximate equivalent exposure if no further information is 
available, in some cases extrapolating doses based on other parameters may be more appropriate.  
This decision should be based on the data available for the individual case.  The body surface 
area normalization and the extrapolation of the animal dose to human dose should be done in one 
step by dividing the NOAEL in each of the animal species studied by the appropriate body 
surface area conversion factor (BSA-CF).  This conversion factor is a unitless number that 
converts mg/kg dose for each animal species to the mg/kg dose in humans, which is equivalent to 
the animals NOAEL on a mg/m2 basis.  The resulting figure is called a human equivalent dose 
(HED).  The species that generates the lowest HED is called the most sensitive species.  
 
When information indicates that a particular species is more relevant for assessing human risk 
(and deemed the most appropriate species), the HED for that species may be used in subsequent 
calculations, regardless of whether this species is the most sensitive.  This situation is more 
applicable to biologic therapies, many of which have high selectivity for binding to human target 
  3 Contains Nonbinding Recommendations 
 
proteins and limited reactivity in species commonly used for toxicity testing.  In such cases, in 
vitro binding and functional studies should be conducted to select an appropriate, relevant 
species before toxicity studies are designed (refer to ICH guidance for industry S6 Preclinical 
Safety Evaluation of Biotechnology-Derived Pharmaceuticals for more details3).  (However, if 
serious toxicities are observed in an animal species considered less relevant, those toxicities 
should be taken into consideration in determining the species to be used to calculate an HED.  
For example, in one particular case, dog was selected as the animal species used for calculation 
of an HED because of unmonitorable cardiac lesions, even though the rat was considered the 
most relevant species based on pharmacological activity data.)  Additionally, a species might be 
considered an inappropriate toxicity model for a given drug if the dose-limiting toxicity in that 
species was concluded to be of limited value for human risk assessment, based on historical 
comparisons of toxicities in the animal species to those in humans across a therapeutic class (i.e., 
the dose-limiting toxicity is species-specific).  In this case, data from that species should not be 
used to derive the HED.  Without any additional information to guide the choice of the most 
appropriate species for assessing human risk, the most sensitive species is designated the most 
appropriate, because using the lowest HED would generate the most conservative starting dose.   
 
A safety factor should then be applied to the HED to increase assurance that the first dose in 
humans will not cause adverse effects.  The use of the safety factor should be based on the 
possibility that humans may be more sensitive to the toxic effects of a therapeutic agent than 
predicted by the animal models, that bioavailability may vary across species, and that the models 
tested do not evaluate all possible human toxicities.  For example, ocular disturbances or pain 
(e.g., severe headaches) in humans can be significant dose-limiting toxicities that may go 
undetected in animal studies.   
 
In general, one should consider using a safety factor of at least 10.  The MRSD should be 
obtained by dividing the HED by the safety factor.  Safety concerns or design shortcomings 
noted in animal studies may increase the safety factor, and thus reduce the MRSD further.  
Alternatively, information about the pharmacologic class (well-characterized classes of 
therapeutics with extensive human clinical and preclinical experience) may allay concerns and 
form the basis for reducing the magnitude of the default safety factor and increasing the MRSD.  
Although a dose lower than the MRSD can be used as the actual starting dose, the process 
described in this guidance will derive the maximum recommended starting dose.  This algorithm 
generates an MRSD in units of mg/kg, a common method of dosing used in phase 1 trials, but the 
equations and conversion factors provided in this guidance (Table 1, second column) can be used 
to generate final dosing units in the mg/m2 form if desired.  
 
As previously stated, for purposes of initial clinical trials in adult healthy volunteers, the HED 
should ordinarily be calculated from the animal NOAEL.  If the HED is based on an alternative 
index of effect, such as the pharmacologically active dose (PAD), this exception should be 
prominently stipulated in descriptions of starting dose calculations. 
 
                                                 
3 We update guidances periodically.  To make sure you have the most recent version of a guidance, check the CDER 
guidance Web page at http://www.fda.gov/cder/guidance/index.htm. 
  4 Contains Nonbinding Recommendations 
 
The remainder of this guidance provides a description of the individual steps in the 
recommended process and the reasoning behind each step.   
 
 
IV.  STEP 1:  NO OBSERVED ADVERSE EFFECT LEVEL DETERMINATION  
 
The first step in determining the MRSD is to review and evaluate the available animal data so 
that a NOAEL can be determined for each study.  Several definitions of NOAEL exist, but for 
selecting a starting dose, the following is used:  the highest dose level that does not produce a 
significant increase in adverse effects in comparison to the control group.  In this context, 
adverse effects that are biologically significant (even if they are not statistically significant) 
should be considered in the determination of the NOAEL.  The NOAEL is a generally accepted 
benchmark for safety when derived from appropriate animal studies and can serve as the starting 
point for determining a reasonably safe starting dose of a new therapeutic in healthy (or 
asymptomatic) human volunteers.   
 
The NOAEL is not the same as the no observed effect level (NOEL), which refers to any effect, 
not just an adverse one, although in some cases the two might be identical.  The definition of the 
NOAEL, in contrast to that of the NOEL, reflects the view that some effects observed in the 
animal may be acceptable pharmacodynamic actions of the therapeutic and may not raise a safety 
concern.  The NOAEL should also not be confused with lowest observed adverse effect level 
(LOAEL) or maximum tolerated dose (MTD).  Both of the latter concepts are based on findings 
of adverse effects and are not generally used as benchmarks for establishing safe starting doses 
in adult healthy volunteers.  (The term level refers to dose or dosage, generally expressed as 
mg/kg or mg/kg/day.)   
 
Initial IND submissions for first-in-human studies by definition lack in vivo human data or 
formal allometric comparison of pharmacokinetics.  Measurements of systemic levels or 
exposure (i.e., AUC or C ) cannot be employed for setting a safe starting dose in humans, and 
max
it is critical to rely on dose and observed toxic response data from adequate and well-conducted 
toxicology studies.  However, there are cases where nonclinical data on bioavailability, 
metabolite profile, and plasma drug levels associated with toxicity may influence the choice of 
the NOAEL.  One such case is when saturation of drug absorption occurs at a dose that produces 
no toxicity.  In this instance, the lowest saturating dose, not the highest (nontoxic) dose, should 
be used for calculating the HED. 
 
There are essentially three types of findings in nonclinical toxicology studies that can be used to 
determine the NOAEL:  (1) overt toxicity (e.g., clinical signs, macro- and microscopic lesions); 
(2) surrogate markers of toxicity (e.g., serum liver enzyme levels); and (3) exaggerated 
pharmacodynamic effects.  Although the nature and extent of adverse effects can vary greatly 
with different types of therapeutics, and it is anticipated that in many instances, experts will 
disagree on the characterization of effects as being adverse or not, the use of NOAEL as a 
benchmark for dose-setting in healthy volunteers should be acceptable to all responsible 
investigators.  As a general rule, an adverse effect observed in nonclinical toxicology studies 
  5 Contains Nonbinding Recommendations 
 
used to define a NOAEL for the purpose of dose-setting should be based on an effect that would 
be unacceptable if produced by the initial dose of a therapeutic in a phase 1 clinical trial 
conducted in adult healthy volunteers. 
 
 
V.  STEP 2:  HUMAN EQUIVALENT DOSE CALCULATION 
 
A.  Conversion Based on Body Surface Area 
 
After the NOAELs in the relevant animal studies have been determined, they are converted to 
HEDs.  A decision should be made regarding the most appropriate method for extrapolating the 
animal dose to the equivalent human dose.  Toxic endpoints for therapeutics administered 
systemically to animals, such as the MTD, are usually assumed to scale well between species 
when doses are normalized to body surface area (i.e., mg/m2) (EPA 1992; Lowe and Davis 
1998).  The basis for this assumption lies primarily with the work of Freireich et al. (1966) and 
Schein et al. (1970).  These investigators reported that, for antineoplastic drugs, doses lethal to 
10 percent of rodents (LD s) and MTDs in nonrodents both correlated with the human MTD 
10
when the doses were normalized to the same administration schedule and expressed as mg/m2.  
Despite the subsequent analyses showing that the MTDs for this set of drugs scale best between 
species when doses are normalized to W0.75 rather than W0.67 (inherent in body surface area 
normalization) (Travis and White 1988; Watanabe et al. 1992), normalization to body surface 
area has remained a widespread practice for estimating an HED based on an animal dose.   
 
An analysis of the affect of the allometric exponent on the conversion of an animal dose to the 
HED was conducted (see Appendix A).  Based on this analysis and on the fact that correcting for 
body surface area increases clinical trial safety by resulting in a more conservative starting dose 
estimate, it was concluded that the approach of converting NOAEL doses to an HED based on 
body surface area correction factors (i.e., W0.67) should be maintained for selecting starting doses 
for initial studies in adult healthy volunteers.  Nonetheless, use of a different dose normalization 
approach, such as directly equating the human dose to the NOAEL in mg/kg, may be appropriate 
in some circumstances.  Deviations from the body surface area approach, when describing the 
conversion of animal dose to HED, should be justified.  The basis for justifying direct mg/kg 
conversion and examples in which other normalization methods are appropriate are described in 
the following subsection. 
 
Although normalization to body surface area is an appropriate method for extrapolating doses 
between species, consistent factors for converting doses from mg/kg to mg/m2 have not always 
been used.  Given that body surface area normalization provides a reasonable approach for 
estimating an HED, the factors used for converting doses for each species should be 
standardized.  Since body surface area varies with W0.67, the conversion factors are dependent on 
the weight of the animals in the studies.  However, analyses conducted to address the effect of 
body weight on the actual BSA-CF demonstrated that a standard factor provides a reasonable 
estimate of the HED over a broad range of human and animal weights (see Appendix B).  The 
conversion factors and divisors shown in Table 1 are therefore recommended as the standard 
  6 Contains Nonbinding Recommendations 
 
values to be used for interspecies dose conversions for NOAELs.  (These factors may also be 
applied when comparing safety margins for other toxicity endpoints (e.g., reproductive toxicity 
and carcinogenicity) when other data for comparison (i.e., AUCs) are unavailable or are 
otherwise inappropriate for comparison.) 
 
Table 1:  Conversion of Animal Doses to Human Equivalent Doses 
Based on Body Surface Area 
  To Convert  To Convert Animal Dose in mg/kg 
  Animal Dose in  to HEDa in mg/kg, Either: 
Species  mg/kg to Dose in  Divide  Multiply 
mg/m2, Multiply  Animal Dose By Animal Dose By 
by k   
m
Human  37  ---  --- 
Child (20 kg)b  25  ---  --- 
Mouse  3  12.3  0.08 
Hamster  5  7.4  0.13 
Rat  6  6.2  0.16 
Ferret  7  5.3  0.19 
Guinea pig  8  4.6  0.22 
Rabbit  12  3.1  0.32 
Dog  20  1.8  0.54 
Primates:       
Monkeysc  12  3.1  0.32 
Marmoset  6  6.2  0.16 
Squirrel monkey  7  5.3  0.19 
Baboon  20  1.8  0.54 
Micro-pig  27  1.4  0.73 
Mini-pig  35  1.1  0.95 
a Assumes 60 kg human.  For species not listed or for weights outside the standard ranges, 
HED can be calculated from the following formula:  
  HED = animal dose in mg/kg x (animal weight in kg/human weight in kg)0.33. 
b This k  value is provided for reference only since healthy children will rarely be volunteers 
m
for phase 1 trials. 
c For example, cynomolgus, rhesus, and stumptail.   
 
B.  Basis for Using mg/kg Conversions 
 
The factors in Table 1 for scaling animal NOAEL to HEDs are based on the assumption that 
doses scale 1:1 between species when normalized to body surface area.  However, there are 
occasions for which scaling based on body weight (i.e., setting the HED (mg/kg) = NOAEL 
(mg/kg)) may be more appropriate.  To consider mg/kg scaling for a therapeutic, the available 
data should show that the NOAEL occurs at a similar mg/kg dose across species.  The following 
circumstances should exist before extrapolating to the HED on a mg/kg basis rather than using 
  7 Contains Nonbinding Recommendations 
 
the mg/m2 approach.  Note that mg/kg scaling will give a twelve-, six-, and twofold higher HED 
than the default mg/m2 approach for mice, rats, and dogs, respectively.  If these circumstances do 
not exist, the mg/m2 scaling approach for determining the HED should be followed as it will lead 
to a safer MRSD. 
 
1.  NOAELs occur at a similar mg/kg dose across test species (for the studies with a given 
dosing regimen relevant to the proposed initial clinical trial).  (However, it should be 
noted that similar NOAELs on a mg/kg basis can be obtained across species because of 
differences in bioavailability alone.) 
 
2.  If only two NOAELs from toxicology studies in separate species are available, one of the 
following should also be true:   
 
  The therapeutic is administered orally and the dose is limited by local toxicities.  
Gastrointestinal (GI) compartment weight scales by W0.94 (Mordenti 1986).  GI 
volume determines the concentration of the therapeutic in the GI tract.  It is then 
reasonable that the toxicity of the therapeutic would scale by mg/kg (W1.0). 
 
  The toxicity in humans (for a particular class) is dependent on an exposure parameter 
that is highly correlated across species with dose on a mg/kg basis.  For example, 
complement activation by systemically administered antisense oligonucleotides in 
humans is believed to be dependent upon C  (Geary et al. 1997).  For some 
max
antisense drugs, the C  correlates across nonclinical species with mg/kg dose and in 
max
such instances mg/kg scaling would be justified.   
 
  Other pharmacologic and toxicologic endpoints also scale between species by mg/kg 
for the therapeutic.  Examples of such endpoints include the MTD, lowest lethal dose, 
and the pharmacologically active dose. 
 
  There is a robust correlation between plasma drug levels (C  and AUC) and dose in 
max
mg/kg. 
 
C.  Other Exceptions to mg/m2 Scaling Between Species 
 
Scaling between species based on mg/m2 is not recommended for the following categories of 
therapeutics: 
 
1.  Therapeutics administered by alternative routes (e.g., topical, intranasal, subcutaneous, 
intramuscular) for which the dose is limited by local toxicities.  Such therapeutics should 
be normalized to concentration (e.g., mg/area of application) or amount of drug (mg) at 
the application site. 
 
2.  Therapeutics administered into anatomical compartments that have little subsequent 
distribution outside of the compartment.  Examples are intrathecal, intravesical, 
  8 Contains Nonbinding Recommendations 
 
intraocular, or intrapleural administration.  Such therapeutics should be normalized 
between species according to the compartmental volumes and concentrations of the 
therapeutic. 
 
3.  Proteins administered intravascularly with M > 100,000 daltons.  Such therapeutics 
r
should be normalized to mg/kg. 
 
 
VI.  STEP 3:  MOST APPROPRIATE SPECIES SELECTION 
 
After the HEDs have been determined from the NOAELs from all toxicology studies relevant to 
the proposed human trial, the next step is to pick one HED for subsequent derivation of the 
MRSD.  This HED should be chosen from the most appropriate species.  In the absence of data 
on species relevance, a default position is that the most appropriate species for deriving the 
MRSD for a trial in adult healthy volunteers is the most sensitive species (i.e., the species in 
which the lowest HED can be identified).   
 
Factors that could influence the choice of the most appropriate species rather than the default to 
the most sensitive species include:  (1) differences in the absorption, distribution, metabolism, 
and excretion (ADME) of the therapeutic between the species, and (2) class experience that may 
indicate a particular animal model is more predictive of human toxicity.  Selection of the most 
appropriate species for certain biological products (e.g., human proteins) involves consideration 
of various factors unique to these products.  Factors such as whether an animal species expresses 
relevant receptors or epitopes may affect species selection (refer to ICH guidance for industry S6 
Preclinical Safety Evaluation of Biotechnology-Derived Pharmaceuticals for more details). 
When determining the MRSD for the first dose of a new therapeutic in humans, absorption, 
distribution, and elimination parameters will not be known for humans.  Comparative 
metabolism data, however, might be available based on in vitro studies.  These data are 
particularly relevant when there are marked differences in both the in vivo metabolite profiles 
and HEDs in animals.  Class experience implies that previous studies have demonstrated that a 
particular animal model is more appropriate for the assessment of safety for a particular class of 
therapeutics.  For example, in the nonclinical safety assessment of the phosphorothioate 
antisense drugs, the monkey is considered the most appropriate species because monkeys 
experience the same dose limiting toxicity as humans (e.g., complement activation) whereas 
rodents do not.  For this class of therapeutics, the MRSD would usually be based on the HED for 
the NOAEL in monkeys regardless of whether it was lower than that in rodents, unless unique 
dose limiting toxicities were observed with the new antisense compound in the rodent species.   
 
 
VII.  STEP 4:  APPLICATION OF SAFETY FACTOR 
 
Once the HED of the NOAEL in the most appropriate species has been determined, a safety 
factor should then be applied to provide a margin of safety for protection of human subjects 
receiving the initial clinical dose.  This safety factor allows for variability in extrapolating from 
  9 Contains Nonbinding Recommendations 
 
animal toxicity studies to studies in humans resulting from:  (1) uncertainties due to enhanced 
sensitivity to pharmacologic activity in humans versus animals; (2) difficulties in detecting 
certain toxicities in animals (e.g., headache, myalgias, mental disturbances); (3) differences in 
receptor densities or affinities; (4) unexpected toxicities; and (5) interspecies differences in 
ADME of the therapeutic.  These differences can be accommodated by lowering the human 
starting dose from the HED of the selected species NOAEL.   
 
In practice, the MRSD for the clinical trial should be determined by dividing the HED derived 
from the animal NOAEL by the safety factor.  The default safety factor that should normally be 
used is 10.  This is a historically accepted value, but, as described below, should be evaluated 
based on available information.  
 
A safety factor of 10 may not be appropriate for all cases.  The safety factor should be raised 
when there is reason for increased concern, and lowered when concern is reduced because of 
available data that provide added assurance of safety.  This can be visualized as a sliding scale, 
balancing findings that mitigate the concern for harm to healthy volunteers with those that 
suggest greater concern is warranted.  The extent of the increase or decrease is largely a matter of 
judgment, using the available information.  It is incumbent on the evaluator to clearly explain the 
reasoning behind the applied safety factor when it differs from the default value of 10, 
particularly if it is less than 10.  
 
A.  Increasing the Safety Factor 
 
The following considerations indicate a safety concern that might warrant increasing the safety 
factor.  In these circumstances, the MRSD would be calculated by dividing the HED by a safety 
factor that is greater than 10.  If any of the following concerns are defined in review of the 
nonclinical safety database, an increase in the safety factor may be called for.  If multiple 
concerns are identified, the safety factor should be increased accordingly.  
 
  Steep dose response curve.  A steep dose response curve for significant toxicities in the 
most appropriate species or in multiple species may indicate a greater risk to humans.  
 
  Severe toxicities.  Qualitatively severe toxicities or damage to an organ system (e.g., 
central nervous system (CNS)) indicate increased risk to humans.  
 
  Nonmonitorable toxicity.  Nonmonitorable toxicities may include histopathologic 
changes in animals that are not readily monitored by clinical pathology markers.   
 
  Toxicities without premonitory signs.  If the onset of significant toxicities is not 
reliably associated with premonitory signs in animals, it may be difficult to know when 
toxic doses are approached in human trials.  
 
  10 Contains Nonbinding Recommendations 
 
  Variable bioavailability.  Widely divergent or poor bioavailability in the several animal 
species, or poor bioavailability in the test species used to derive the HED, suggest a 
greater possibility for underestimating the toxicity in humans.  
 
  Irreversible toxicity.  Irreversible toxicities in animals suggest the possibility of 
permanent injury in human trial participants.  
 
  Unexplained mortality.  Mortality that is not predicted by other parameters raises the 
level of concern. 
 
  Large variability in doses or plasma drug levels eliciting effect.  When doses or 
exposure levels that produce a toxic effect differ greatly across species or among 
individual animals of a species, the ability to predict a toxic dose in humans is reduced 
and a greater safety factor may be needed.  
 
  Nonlinear pharmacokinetics.  When plasma drug levels do not increase in a dose-
related manner, the ability to predict toxicity in humans in relation to dose is reduced and 
a greater safety factor may be needed. 
 
  Inadequate dose-response data.  Poor study design (e.g., few dose levels, wide dosing 
intervals) or large differences in responses among animals within dosing groups may 
make it difficult to characterize the dose-response curve.  
 
  Novel therapeutic targets.  Therapeutic targets that have not been previously clinically 
evaluated may increase the uncertainty of relying on the nonclinical data to support a safe 
starting dose in humans. 
 
  Animal models with limited utility.  Some classes of therapeutic biologics may have 
very limited interspecies cross-reactivity or pronounced immunogenicity, or may work by 
mechanisms that are not known to be conserved between (nonhuman) animals and 
humans; in these cases, safety data from any animal studies may be very limited in scope 
and interpretability. 
 
B.  Decreasing the Safety Factor 
 
Safety factors of less than 10 may be appropriate under some conditions.  The toxicologic testing 
in these cases should be of the highest caliber in both conduct and design.  Most of the time, 
candidate therapeutics for this approach would be members of a well-characterized class.  Within 
the class, the therapeutics should be administered by the same route, schedule, and duration of 
administration; should have a similar metabolic profile and bioavailability; and should have 
similar toxicity profiles across all the species tested including humans.  A smaller safety factor 
might also be used when toxicities produced by the therapeutic are easily monitored, reversible, 
predictable, and exhibit a moderate-to-shallow dose-response relationship with toxicities that are 
  11 Contains Nonbinding Recommendations 
 
consistent across the tested species (both qualitatively and with respect to appropriately scaled 
dose and exposure). 
 
A safety factor smaller than 10 could be justified when the NOAEL was determined based on 
toxicity studies of longer duration compared to the proposed clinical schedule in healthy 
volunteers.  In this case, a greater margin of safety should be built into the NOAEL, as it was 
associated with a longer duration of exposure than that proposed in the clinical setting.  This 
assumes that toxicities are cumulative, are not associated with acute peaks in therapeutic 
concentration (e.g., hypotension), and did not occur early in the repeat dose study.  
 
 
VIII.  STEP 5:  CONSIDERATION OF THE PHARMACOLOGICALLY ACTIVE 
DOSE  
 
Selection of a PAD depends upon many factors and differs markedly among pharmacological 
drug classes and clinical indications; therefore, selection of a PAD is beyond the scope of this 
guidance.  However, once the MRSD has been determined, it may be of value to compare it to 
the PAD derived from appropriate pharmacodynamic models.  If the PAD is from an in vivo 
study, an HED can be derived from a PAD estimate by using a BSA-CF.  This HED value should 
be compared directly to the MRSD.  If this pharmacologic HED is lower than the MRSD, it may 
be appropriate to decrease the clinical starting dose for pragmatic or scientific reasons.  
Additionally, for certain classes of drugs or biologics (e.g., vasodilators, anticoagulants, 
monoclonal antibodies, or growth factors), toxicity may arise from exaggerated pharmacologic 
effects.  The PAD in these cases may be a more sensitive indicator of potential toxicity than the 
NOAEL and might therefore warrant lowering the MRSD.  
 
 
IX.  SUMMARY 
 
A strategy has been proposed to determine the maximum recommended starting dose for clinical 
trials of new therapeutics in adult healthy volunteers.  In summary, usually NOAELs from the 
relevant animal studies should be converted to the HEDs using the standard factors presented in 
Table 1.  Using sound scientific judgment, a safety factor should be applied to the HED from the 
most appropriate species to arrive at the MRSD.  This process is meant to define the upper limit 
of recommended starting doses and, in general, lower starting doses can be appropriate.  The 
process described in this guidance should foster consistency among sponsors and Agency 
reviewers.  
 
 
  12 Contains Nonbinding Recommendations 
 
REFERENCES 
 
Boxenbaum, H and C DiLea, 1995, First-Time-in-Human Dose Selection:  Allometric Thoughts 
and Perspectives, Journal of Clinical Pharmacology, 35:957-966. 
 
Burtles, SS, DR Newell, REC Henrar, and TA Connors, 1995, Revisions of General Guidelines 
for the Preclinical Toxicology of New Cytotoxic Anticancer Agents in Europe, European Journal 
of Cancer, 31A:408-410. 
 
Contrera, JF, AC Jacobs, RP Hullahalli, M Mehta, WJ Schmidt, and JA DeGeorge, 1995, 
Systemic Exposure  Based Alternative to the Maximum Tolerated Dose for Carcinogenicity 
Studies of Human Therapeutics, Journal of American College of Toxicology, 14:1-10. 
 
EPA, 1992, A Cross-Species Scaling Factor for Carcinogen Risk Assessment Based on 
Equivalence of Mg/Kg0.75/Day, Federal Register, 57:24152-24173. 
 
Freireich, EJ, EA Gehan, DP Rall, LH Schmidt, and HE Skipper, 1966, Quantitative Comparison 
of Toxicity of Anticancer Agents in Mouse, Rat, Hamster, Dog, Monkey, and Man, Cancer 
Chemotherapy Reports, 50:219-244. 
 
Geary, RS, JM Leeds, SP Henry, DK Monteith, and AA Levin, 1997, Antisense Oligonucleotide 
Inhibitors for the Treatment of Cancer:  1. QUESTION Pharmacokinetic Properties of 
Phosphorothioate Oligodeoxynucleotides, Anti-Cancer Drug Design, 12:383-393. 
 
Lowe, MC and RD Davis, 1998, The Current Toxicology Protocol of the National Cancer 
Institute, in K Hellman and SK Carter (eds.), Fundamentals of Cancer Chemotherapy, pp. 228-
235, New York: McGraw Hill.   
 
Mahmood, I, MD Green, and JE Fisher, 2003, Selection of the First-Time Dose in Humans:  
Comparison of Different Approaches Based on Interspecies Scaling of Clearance, 43(7):692-697. 
 
Mordenti, J, 1986, Man Vs. Beast:  Pharmacokinetic Scaling in Mammals, Journal of 
Pharmaceutical Sciences, 75:1028-1040. 
 
Reigner, BG and KS Blesch, 2002, Estimating the Starting Dose for Entry into Humans:  
Principles and Practice, European Journal of Clinical Pharmacology, 57:835-845. 
 
Schein, PS, RD Davis, S Carter, J Newman, DR Schein, and DP Rall, 1970, The Evaluation of 
Anticancer Drugs in Dogs and Monkeys for the Prediction of Qualitative Toxicities in Man, 
Clinical Pharmacology and Therapeutics, 11:3-40. 
 
Spector, WS (ed.), 1956, Handbook of Biological Data, pp. 175, Philadelphia: W.B. Saunders 
Co. 
 
  13 Contains Nonbinding Recommendations 
 
Stahl, WR, 1967, Scaling of Respiratory Variables in Mammals, Journal of Applied Physiology, 
22:453-460. 
 
Travis, CC and RK White, 1988, Interspecies Scaling of Toxicity Data, Risk Analysis, 8:119-
125. 
 
Watanabe, K, FY Bois, and L Zeise, 1992, Interspecies Extrapolation:  A Reexamination of 
Acute Toxicity Data, Risk Analysis, 12:301-310. 
 
International Conference on Harmonisation Guidances  
 
ICH guidance for industry S6 Preclinical Safety Evaluation of Biotechnology-Derived 
Pharmaceuticals  
 
ICH guidance for industry S3A Toxicokinetics:  The Assessment of Systemic Exposure in Toxicity 
Studies 
 
ICH guidance for industry M3 Nonclinical Safety Studies for the Conduct of Human Clinical 
Trials for Pharmaceuticals 
 
 
  14 Contains Nonbinding Recommendations 
 
GLOSSARY 
 
b:  Allometric exponent 
Body surface area conversion factor (BSA-CF):  A factor that converts a dose (mg/kg) in an 
animal species to the equivalent dose in humans (also known as the human equivalent dose), 
based on differences in body surface area.  A BSA-CF is the ratio of the body surface areas in the 
tested species to that of an average human. 
 
Human equivalent dose (HED):  A dose in humans anticipated to provide the same degree of 
effect as that observed in animals at a given dose.  In this guidance, as in many communications 
from sponsors, the term HED is usually used to refer to the human equivalent dose of the 
NOAEL.  When reference is made to the human equivalent of a dose other than the NOAEL 
(e.g., the PAD), sponsors should explicitly and prominently note this usage.  
 
K:  A dimensionless factor that adjusts for differences in the surface area to weight ratio of 
species because of their different body shapes. 
 
k :  Factor for converting mg/kg dose to mg/m2 dose 
m
 
Lowest observed adverse effect level (LOAEL):  The lowest dose tested in an animal species 
with adverse effects. 
 
Maximum recommended starting dose (MRSD):  The highest dose recommended as the initial 
dose in a clinical trial.  In clinical trials of adult healthy volunteers, the MRSD is predicted to 
cause no adverse reactions.  The units of the dose (e.g., mg/kg or mg/m2) may vary depending on 
practices employed in the area being investigated. 
 
Maximum tolerated dose (MTD):  In a toxicity study, the highest dose that does not produce 
unacceptable toxicity. 
 
No observed adverse effect level (NOAEL):  The highest dose tested in an animal species that 
does not produce a significant increase in adverse effects in comparison to the control group.  
Adverse effects that are biologically significant, even if not statistically significant, should be 
considered in determining an NOAEL. 
 
No observed effect level (NOEL):  The highest dose tested in an animal species with no 
detected effects. 
 
Pharmacologically active dose (PAD):  The lowest dose tested in an animal species with the 
intended pharmacologic activity. 
 
Safety factor (SF):  A number by which the HED is divided to introduce a margin of safety 
between the HED and the maximum recommended starting dose. 
 
W:  Body weight in kg 
  15 Contains Nonbinding Recommendations 
 
APPENDIX A: 
Analysis of Allometric Exponent on HED Calculations 
 
An analysis was conducted to determine the effect of the allometric exponent on the conversion 
of an animal dose to the HED.  One can derive the following equation (see Appendix C) for 
converting animal doses to the HED based on body weights and the allometric exponent (b): 
 
  HED = animal NOAEL x (W /W )(1-b) 
animal human
 
Conventionally, for a mg/m2 normalization b would be 0.67, but a number of studies (including 
the original Freireich data) have shown that MTDs scale best across species when b = 0.75.  The 
Interagency Pharmacokinetics Group has recommended that W0.75 be used for interspecies 
extrapolation of doses in carcinogenicity studies (EPA 1992).  There are no data, however, to 
indicate the optimal method for converting NOAELs to HEDs.  Conversion factors were 
calculated over a range of animal and human weights using (W /W )0.33 or 
animal human
(W /W )0.25 to assess the effect on starting dose selection of using b = 0.75 instead of b = 
animal human
0.67.  The results are shown in Table 2.  Using an allometric exponent of 0.75 had a big effect on 
the conversion factor for the smaller species mice and rats.  Nonetheless, mice are not commonly 
used for toxicology studies to support the first-in-human clinical trials.  In addition, there is 
evidence that the area under the plasma concentration versus time curves in rats and humans 
correlates reasonably well when doses are normalized to mg/m2 (Contrera et al. 1995).  We 
conclude that the approach of converting NOAEL doses to an HED based on body surface area 
correction factors (i.e., b = 0.67) should be maintained for selecting starting doses for initial 
studies in healthy volunteers since:  (1) mg/m2 normalization is widely used throughout the 
toxicology and pharmacokinetic research communities; (2) mg/m2 normalization provides a more 
conservative conversion; (3) there are no data to suggest a superior method for converting 
NOAELs; and (4) CDER has significant experience in establishing safe starting doses based on 
mg/m2, and it is readily calculated. 
 
 
Table 2:  Effect of Allometric Exponent on Conversion Factora 
       
    Conversion Factorsc  Ratio of 
0.75 to 0.67 
         
Species  Weight Rangeb  Standard  b = 0.67  b = 0.75   
(kg) 
           
Mouse  0.018-0.033   0.081 0.075 0.141 1.88 
           
Rat  0.09-0.40  0.162 0.156 0.245 1.57 
           
Rabbit  1.5-3  0.324 0.33 0.43 1.30 
           
Monkey  1.5-4  0.324 0.37 0.47 1.27 
           
Dog  6.5-13.0  0.541 0.53 0.62 1.17 
a conversion factor = (W /W )(1-b) 
animal human
b human weight range used was 50-80 kg (110-176 lb) 
c mean conversion factor calculated across entire animal weight range and human weight 
range 
 
  16 Contains Nonbinding Recommendations 
 
The following summarizes the analysis of the effects of the allometric exponent on HED 
calculations: 
 
  Changing the allometric exponent from 0.67 to 0.75 had a big effect on the conversion 
factor for the smaller rodent species; for mice the conversion factors differed by a factor 
of almost 2.  
 
  Converting doses based on an exponent of 0.75 would lead to higher, more aggressive 
and potentially more toxic starting doses.   
 
  The limited data available suggest that the most accurate allometric exponent for 
normalizing MTDs of antineoplastic agents for interspecies extrapolation is b = 0.75, but 
there are no data to indicate the optimal normalization method for interspecies 
extrapolation of NOAELs in a broad range of therapeutic classes.  Using mg/m2 is widely 
adopted throughout the drug development community.   
 
  Unless evidence is provided to the contrary, HED calculations should be based on b = 
0.67 (i.e., the standard conversions based on mg/m2 relationships). 
 
  There was no notable effect of body weight on calculation of the HED within the weight 
ranges examined.  
 
 
  17 Contains Nonbinding Recommendations 
 
APPENDIX B: 
Analysis of Body Weight Effects on HED Calculations 
 
Accurate conversion of a mg/kg dose to a mg/m2 dose depends on the actual weight (and surface 
area) of the test species.  A popular formula for converting doses is: 
 
(i)  mg/m2 = k   mg/kg 
m
where  k  = 100/K  W0.33 where K is a value unique to each species (Freireich et  
m
  al. 1966) 
or         k  = 9.09  W0.35 where a K value unique to each species is not 
m
needed (Boxenbaum and DiLea 1995; Burtles et al. 1995; Stahl 1956).  
 
The k  value is not truly constant for any species, but increases within a species as body weight 
m
increases.  The increase is not linear, but increases approximately proportional to W2/3.  For 
example, the k  value in rats varies from 5.2 for a 100 g rat to 7.0 for a 250 g rat.  Strictly 
m
speaking, the k  value of 6 applies only to rats at the reference weight of 150 g.  For 
m
standardization and practical purposes, a fixed k  factor for each species is preferred.  An 
m
analysis was undertaken to determine the effect of different body weights within a species on the 
conversion of an animal dose to the HED using k  factors.  The k  factor was calculated for a 
m m
range of body weights using k  = 100/K  W0.33.  In Table 3, a working weight range is shown 
m
next to the reference body weight.  This is the range within which the HED calculated by using 
the standard k  value will not vary more than 20 percent from that which would be calculated 
m
using a k  value based on exact animal weight.  This is a relativity small variance considering 
m
dose separation generally used in deriving the NOAEL, in toxicology studies, which are often 
twofold separations.  For example, suppose a NOAEL in rats is 75 mg/kg and the average rat 
weight is 250 g.  The k  value for a 250 g rat is 7.0.   
m
 
    HED = 75  (7/37) = 14 mg/kg in humans.   
  Using the standard k  value of 6 for rats, 
m
    HED = 75  (6/37) = 12 mg/kg in humans. 
 
The HED calculated with the standard k  value of 6 is within 15 percent of the value calculated 
m
using the actual k  value of 7.  As shown in Table 3, the body weights producing k  factors for 
m m
which the nominal, integer conversion factor was within 20 percent of the calculated factor 
covered a broad range.  This working weight range encompassed the animal weights expected 
for the majority of studies used to support starting doses in humans. 
 
 
 
  18 Contains Nonbinding Recommendations 
 
 
Table 3:  Conversion of Animal Doses to Human Equivalent Doses Based on Body Surface Area 
        To Convert Dose in   To Convert Animal Dose in mg/kg 
  Reference  Working Weight  Body Surface  mg/kg to Dose in  to HEDb in mg/kg, Either 
Species  Body Weight  Rangea (kg)  Area (m2)  mg/m2 Multiply by k   Divide  Multiply 
m
(kg)  Animal Dose By Animal Dose By
Human  60  ---  1.62  37  ---  --- 
Childc  20  ---  0.80  25  ---  --- 
Mouse  0.020  0.011-0.034  0.007  3  12.3  0.081 
Hamster  0.080  0.047-0.157  0.016  5  7.4  0.135 
Rat  0.150  0.080-0.270  0.025  6  6.2  0.162 
Ferret  0.300  0.160-0.540  0.043  7  5.3  0.189 
Guinea pig  0.400  0.208-0.700  0.05  8  4.6  0.216 
Rabbit  1.8  0.9-3.0  0.15  12  3.1  0.324 
Dog  10  5-17  0.50  20  1.8  0.541 
Primates:             
Monkeysd  3  1.4-4.9  0.25  12  3.1  0.324 
Marmoset  0.350  0.140-0.720  0.06  6  6.2  0.162 
Squirrel monkey  0.600  0.290-0.970  0.09  7  5.3  0.189 
Baboon  12  7-23  0.60  20  1.8  0.541 
Micro-pig  20  10-33  0.74  27  1.4  0.730 
Mini-pig  40  25-64  1.14  35  1.1  0.946 
a For animal weights within the specified ranges, the HED for a 60 kg human calculated using the standard k  value will not vary more than 20 percent 
m
from the HED calculated using a k  value based on the exact animal weight. 
m
b Assumes 60 kg human.  For species not listed or for weights outside the standard ranges, human equivalent dose can be calculated from the formula:  
HED = animal dose in mg/kg x (animal weight in kg/human weight in kg)0.33. 
c The k  value is provided for reference only since healthy children will rarely be volunteers for phase 1 trials. 
m
d For example, cynomolgus, rhesus, and stumptail.
  19 Contains Nonbinding Recommendations 
 
For the typical species used in nonclinical safety studies, Table 3 also shows the body surface 
area in m2 for an animal at a particular reference weight.  For example, a 400 g guinea pig has a 
body surface area of approximately 0.05 m2.  These values come from published sources with 
surface area determined experimentally by various methods.  Compilations of this type of data 
can be found in published references (Spector 1956).  
 
For animal weights outside the working weight range in Table 3, or for species not included in 
the table, an alternative method is available for calculating the HED.  In these cases the 
following formula can be used: 
 
HED = Animal dose (mg/kg)  [animal weight (kg)  human weight (kg)]0.33  
 
For example, assume that a NOAEL of 25 mg/kg was determined in a study using rabbits 
weighing 4.0 kg.  The 4.0 kg animals are outside the working range for rabbits of 0.9 to 3.0 kg 
indicated in Table 3. 
 
HED = 25 mg/kg  (4.0  60)0.33 = 25  (0.41) = 10 mg/kg  
 
Alternatively, if the standard conversion factor was used to calculate the HED 
 
HED = 25 mg/kg  3.1 = 8.1 mg/kg 
 
The value of 10 mg/kg for the HED is 25 percent greater than the value of 8.1 mg/kg that would 
be calculated using the standard conversion factor.  For example, assume that a NOAEL of 25 
mg/kg was determined in a study using rabbits weighing 4.0 kg.  The 4.0 kg animals are outside 
the working range for rabbits of 0.9 to 3.0 kg indicated in Table 3. 
 
HED = 25 mg/kg  (4.0  60)0.33 = 25  (0.41) = 10 mg/kg  
 
Alternatively, if the standard conversion factor was used to calculate the HED 
 
HED = 25 mg/kg  3.1 = 8.1 mg/kg 
 
The value of 10 mg/kg for the HED is 25 percent greater than the value of 8.1 mg/kg that would 
be calculated using the standard conversion factor. 
 
The k  analysis addresses only half of the HED conversion process.  The range of human sizes 
m
should also be considered to convert the mg/m2 dose back to an HED dose in mg/kg.  To 
examine the effect of both animal and human weights on the conversion factor, the principle of 
allometry was used.  Interspecies biologic parameters are often related by the power function Y = 
aWb where W is body weight and b (allometric exponent) is the slope of the log-log plot, logy = 
b  logW + C.  Using algebraic manipulation (see Appendix C), one can derive an equation for 
converting an animal dose to the HED based on the body weights of the human and the animals 
for a given allometric exponent.  For converting an animal NOAEL in mg/kg to the HED in 
mg/kg, the equation is: 
 
  20 Contains Nonbinding Recommendations 
 
(ii)  HED = animal NOAEL x (W /W )(1-b) 
animal human
 
Since body surface area is believed to scale with an allometric exponent (b) of 0.67, one can 
explore how the animal and human body weights affect the conversion factor 
(W /W )0.33. 
animal human
 
The conversion factor was calculated over a range of animal weights and a range of human 
weights from 50-80 kg.  The results are summarized in Table 4.  Column B is the weight range 
of the animals used to calculate, in conjunction with the 50-80 kg range in humans, the 
conversion factor.  The extremes of the conversion factors for the permutations chosen are 
shown in columns C and D.  The proposed standard conversion factors are shown in column E.  
The percentage difference of these extremes from the standard is shown in column F.  Finally, 
the range of animal weights that produced a conversion factor for a 60 kg human within 20 
percent of the standard factor is shown in column G.  The 10 percent and 20 percent intervals 
across the entire range of weights are graphically illustrated for rats in Table 5. 
 
 
Table 4:  Effect of Body Weight on Human Equivalent Dose Conversionsa 
             
A  B  C  D  E  F  G 
            
      Conv ersion Facto rc  % Difference  20% Rangef for 
Species  Animal  sm animal  lg animal  Standardd of Extremee  60 kg Human  
Weight  lg human  sm human from  (kg) 
Rangeb (kg)  Standard 
             
Mouse  0.018-0.033  0.060  0.089  0.081  -22%  0.015-0.051 
             
Rat  0.090-0.400  0.106  0.213  0.162  -35%  0.123-0.420 
             
Rabbit  1.5-3.0  0.269  0.395  0.324  +22%  1.0-3.4 
             
Monkey  1.5-4.0  0.319  0.435  0.324  +34%  1.0-3.4 
             
Dog  6.5-13.0  0.437  0.641  0.541  -19%  4.7-16.2 
a conversion factor = (W /W )0.33 
animal human
b human weight range used was 50-80 kg (110-176 lb) 
c HED in mg/kg equals animal dose in mg/kg multiplied by this value 
d See Table 1 
e extreme from column C or D 
f range of animal weights that produced a calculated conversion factor within 20 percent of the standard 
factor (column E) when human weight was set at 60 kg 
 
 
  21 Contains Nonbinding Recommendations 
 
Table 5:  Human and Rat Body Weights Producing Body Surface Area Dose Conversion Factors 
Within 10 Percent and 20 Percent of the Standard Factor (0.162) 
 
EFFECT OF BODY WEIGHT ON BSA-CF 
HED = animal NOAEL (W /W )exp(1-b), b = 0.67 for mg/m2 conversion 
animal human
                  Standard conversion to mg/kg = 0.162   10%  0.146-0.178   
 20%  0.130-0.194   
Rat Body  Human Body Weight (kg)  
Weight (kg)  50  55  60  65  70  75  80 
0.090  0.124  0.120  0.117  0.114  0.111  0.109  0.106 
0.100  0.129  0.125  0.121  0.118  0.115  0.113  0.110 
0.110  0.133  0.129  0.125  0.122  0.119  0.116  0.114 
0.120  0.137  0.132  0.129  0.125  0.122  0.119  0.117 
0.130  0.140  0.136  0.132  0.129  0.126  0.123  0.120 
0.140  0.144  0.139  0.135  0.132  0.129  0.126  0.123 
0.150  0.147  0.142  0.138  0.135  0.132  0.129  0.126 
0.160  0.150  0.146  0.141  0.138  0.134  0.131  0.129 
0.170  0.153  0.149  0.144  0.141  0.137  0.134  0.131 
0.180  0.156  0.151  0.147  0.143  0.140  0.137  0.134 
0.190  0.159  0.154  0.150  0.146  0.142  0.139  0.136 
0.200  0.162  0.157  0.152  0.148  0.145  0.141  0.138 
0.210  0.164  0.159  0.155  0.151  0.147  0.144  0.141 
0.220  0.167  0.162  0.157  0.153  0.149  0.146  0.143 
0.230  0.169  0.164  0.159  0.155  0.152  0.148  0.145 
0.240  0.172  0.166  0.162  0.157  0.154  0.150  0.147 
0.250  0.174  0.169  0.164  0.160  0.156  0.152  0.149 
0.260  0.176  0.171  0.166  0.162  0.158  0.154  0.151 
0.270  0.179  0.173  0.168  0.164  0.160  0.156  0.153 
0.280  0.181  0.175  0.170  0.166  0.162  0.158  0.155 
0.290  0.183  0.177  0.172  0.168  0.164  0.160  0.157 
0.300  0.185  0.179  0.174  0.179  0.165  0.162  0.158 
0.310  0.187  0.181  0.176  0.171  0.167  0.163  0.160 
0.320  0.189  0.183  0.178  0.173  0.169  0.165  0.162 
0.330  0.191  0.185  0.180  0.175  0.171  0.167  0.163 
0.340  0.193  0.187  0.181  0.177  0.172  0.169  0.165 
0.350  0.194  0.188  0.183  0.178  0.174  0.170  0.167 
0.360  0.196  0.190  0.185  0.180  0.176  0.172  0.168 
0.370  0.198  0.192  0.187  0.182  0.177  0.173  0.170 
0.380  0.200  0.194  0.188  0.183  0.179  0.175  0.171 
0.390  0.202  0.195  0.190  0.185  0.180  0.176  0.173 
0.400  0.203  0.197  0.191  0.186  0.182  0.178  0.174 
0.410  0.205  0.199  0.193  0.188  0.183  0.179  0.175 
0.420  0.207  0.200  0.194  0.189  0.185  0.181  0.177 
0.430  0.208  0.202  0.196  0.191  0.186  0.182  0.178 
0.440  0.210  0.203  0.197  0.192  0.188  0.183  0.180 
0.450  0.211  0.205  0.199  0.194  0.189  0.185  0.181 
0.460  0.213  0.206  0.200  0.195  0.190  0.186  0.182 
 
 
  22 Contains Nonbinding Recommendations 
 
The following are conclusions from these analyses: 
  The 20 percent interval around the standard conversion factor includes a broad range of 
animal and human weights. 
 
  Given that the human weights will vary broadly, it is not usually necessary to be 
concerned about the affect of the variation of animal weights within a species on the 
HED calculation. 
 
  If an extreme animal weight is encountered in a toxicology study, one can calculate an 
accurate conversion factor using (W /W )0.33. 
animal human
 
 
  23 Contains Nonbinding Recommendations 
 
APPENDIX C: 
Derivation of the Interspecies Scaling Factor (W /W )(1-b) 
a h
 
Power equation   (mg) = aWb 
log(mg) = log(a) + bClog(W) = bClog(W) + c 
 
Given the weights of animal and human, and animal dose in mg/kg, solve for HED in mg/kg: 
Let  H = mg/kg dose in humans 
A = mg/kg dose in animals 
W  = weight of human 
h
W  = weight of animal 
a
 
  for animal   log(mg) = log(a) + bClog(W ) = bClog(W ) + c  
a a
replace mg  log(ACW ) = bClog(W ) + c 
a a
solve for c  c = log(ACW ) - bClog(W ) 
a a
  = log(A) + log(W ) - bClog(W ) 
a a
  = log(A) + (1-b)log(W ) 
a
 
likewise for human  c = log(H) + (1-b)log(W ) 
h
 
equate two equations   log(A) + (1-b)log(W ) = log(H) + (1-b)log(W ) 
a h
solve for log(H)   log(H)  = log(A) + (1-b)log(W ) - (1-b)log(W ) 
a h
= log(A) + (1-b)[log(W ) - log(W )] 
a h
= log(A) + log[(W /W )(1-b)] 
a h
log(H)  = log[AC(W /W )(1-b)] 
a h
 
solve for H    H = AC(W /W )(1-b) 
a h
 
For example, using mg/m2 normalization (b = 0.67) the predicted human MTD in mg/kg based 
on a rat LD  in mg/kg is MTD = LD  C(W /W )0.33. 
10 10 a h
 
Likewise the HED in mg/kg based on a surface area conversion given an animal NOAEL is  
HED = NOAEL C(W /W )0.33. 
a h
 
 
  24 Contains Nonbinding Recommendations 
 
APPENDIX D: 
Examples of Calculations for Converting Animal Doses 
to Human Equivalent Doses 
 
This appendix provides examples of specific calculations to be taken in deriving an HED based 
on standardized factors. 
 
Tables 1 and 3 provide standardized conversion factors for changing animal or human doses 
expressed as mg/kg to doses expressed as mg/m2.  Tables 1 and 3 also have factors (and divisors) 
for converting animal doses in mg/kg to the human dose in mg/kg that is equivalent to the animal 
dose if both were expressed on a mg/m2 basis.  This human dose in mg/kg is referred to as the 
HED. 
 
 
Example 1:  Converting to mg/m2 HED 
 
To convert an animal or human dose from mg/kg to mg/m2, the dose in mg/kg is multiplied by 
the conversion factor indicated as k  (for mass constant).  The k  factor has units of kg/m2; it is 
m m
equal to the body weight in kg divided by the surface area in m2. 
 
  formula:            mg/kg  k  = mg/m2 
m
  to convert a dose of 30 mg/kg in a dog:    30  20 = 600 mg/m2 
  to convert a dose of 2.5 mg/kg in a human:    2.5  37 = 92.5 mg/m2 
 
 
Example 2:  Converting to mg/kg HED in two steps 
 
To calculate the HED for a particular dose in animals, one can calculate the animal dose in 
mg/m2 by multiplying the dose in mg/kg by the k  factor for that species as described in 
m
Example 1.  The dose can then be converted back to mg/kg in humans by dividing the dose in 
mg/m2 by the k  factor for humans.   
m
 
  formula:  (animal mg/kg dose  animal k )  human k  = human mg/kg dose 
m m
  to calculate the HED for a 15 mg/kg dose in dogs: 
    (15  20)  37 = 300 mg/m2  37 = 8 mg/kg 
 
 
Example 3:  Converting to mg/kg HED in one step 
 
The calculation in Example 2 can be simplified by combining the two steps.  The HED can be 
calculated directly from the animal dose by dividing the animal dose by the ratio of the 
human/animal k  factor (third column in Table 1) or by multiplying by the ratio of the 
m
animal/human k  factor (fourth column in Table 1).   
m
 
  25 Contains Nonbinding Recommendations 
 
Division method 
    NOAEL       calculation      HED      
            mg/kg  [k /k ] 
mhuman manimal
    15 mg/kg in dogs    15 mg/kg  1.8 =     8 mg/kg 
    50 mg/kg in rats    50 mg/kg  6.2 =     8 mg/kg 
    50 mg/kg in monkeys   50 mg/kg  3.1 =     16 mg/kg  
 
 
Multiplication method 
    NOAEL       calculation      HED      
            mg/kg  [k /k ] 
manimal mhuman
    15 mg/kg in dogs    15 mg/kg  0.541 =    8 mg/kg 
    50 mg/kg in rats    50 mg/kg  0.162 =    8 mg/kg 
    50 mg/kg in monkeys   50 mg/kg  0.324 =    16 mg/kg 
 
 
  26 Contains Nonbinding Recommendations 
 
APPENDIX E: 
Selection of Maximum Recommended Starting Dose 
for Drugs Administered Systemically to Normal Volunteers 
 
 
 
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