Testicular Toxicity: 
Evaluation During Drug 
Development 
Guidance for Industry 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Drug Evaluation and Research (CDER) 
 
October 2018 
Clinical/Medical 
Pharmacology/Toxicology 
   
Testicular Toxicity: 
Evaluation During Drug 
Development 
Guidance for Industry 
 
Additional copies are available from: 
 
Office of Communications, Division of Drug Information 
Center for Drug Evaluation and Research 
Food and Drug Administration 
10001 New Hampshire Ave., Hillandale Bldg., 4th Floor  
Silver Spring, MD  20993-0002  
Phone: 855-543-3784 or 301-796-3400; Fax: 301-431-6353; Email: druginfo@fda.hhs.gov 
https://www.fda.gov/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/default.htm 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Drug Evaluation and Research (CDER) 
 
October 2018 
Clinical/Medical 
Pharmacology/Toxicology
   
TABLE OF CONTENTS 
 
I.  INTRODUCTION............................................................................................................. 1 
II.  DIFFICULTIES EVALUATING TESTICULAR TOXICITY IN HUMANS ........... 2 
III.  NONCLINICAL EVALUATION.................................................................................... 2 
A.  Introduction .................................................................................................................................... 2 
B.  Nonclinical Study Design Considerations .................................................................................... 3 
C.  Nonclinical Findings That Raise Concern for Male Fertility ..................................................... 4 
D.  Confounding Factors ..................................................................................................................... 5 
E.  Follow-Up Investigations ............................................................................................................... 5 
IV.  MONITORING OF THE TESTES DURING CLINICAL TRIALS ........................... 6 
V.  DESIGN OF A CLINICAL TRIAL TO EVALUATE THE EFFECT OF A DRUG  
  ON THE TESTES ............................................................................................................. 7 
A.  Subject Selection ............................................................................................................................ 7 
B.  Trial Design .................................................................................................................................... 8 
C.  Presentation of Results .................................................................................................................. 9 
D.  Conclusions ................................................................................................................................... 11 
 
  Contains Nonbinding Recommendations 
Testicular Toxicity: 
Evaluation During Drug Development 
Guidance for Industry1 
 
 
 
This guidance represents the current thinking of the Food and Drug Administration (FDA or Agency) on 
this topic.  It does not establish any rights for any person and is not binding on FDA or the public.  You 
can use an alternative approach if it satisfies the requirements of the applicable statutes and regulations.  
To discuss an alternative approach, contact the FDA office responsible for this guidance as listed on the 
title page.   
 
 
 
I.  INTRODUCTION 
 
The purpose of this guidance is to assist sponsors who are developing drug products that may 
have potential adverse effects on the testes, which we refer to as testicular toxicity, based on 
findings in nonclinical studies.2  This guidance discusses the following topics: 
 
  Nonclinical findings that suggest risk of clinical testicular toxicity, and further 
nonclinical assessments that may be necessary to evaluate the extent of this risk  
 
  Clinical monitoring that can be employed when these drug products are initially 
administered to men 
 
  The design of a clinical trial that has as its primary purpose the evaluation of drug-related 
testicular toxicity 
 
The guidance provides general considerations for when clinical trials to assess the risk of 
testicular toxicity may be needed but does not cover all possible scenarios that would prompt 
such a trial.  The guidance also does not discuss the regulatory actions that FDA might consider 
based on the results of the clinical trials. 
 
In general, FDAs guidance documents 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 Division of Bone, Reproductive, and Urologic Products in the Center for 
Drug Evaluation and Research at the Food and Drug Administration. 
 
2 For the purposes of this guidance, all references to drugs and drug products include both human drugs and 
therapeutic biological products unless otherwise specified. 
1  Contains Nonbinding Recommendations 
 
II.  DIFFICULTIES EVALUATING TESTICULAR TOXICITY IN HUMANS 
 
A thorough evaluation of a drug products testicular toxicity in humans is challenging for the 
following reasons: 
 
  Only a few clinical markers can reliably monitor potential changes in human testicular 
function that might accompany drug exposure.  Examples of measurements of testicular 
function include semen analysis, serum testosterone concentrations, and serum 
gonadotropin concentrations.  
 
  Monitoring for adverse testicular effects in humans in real time presents a challenge 
because a latency period of several months exists between the time of an injury to 
seminiferous tubules and the time when that injury can be detected using the most 
commonly used test:  semen analysis. 
 
  The ability to interpret changes from baseline in the previously mentioned measurements 
of testicular function and to correlate those changes with effects on male fertility is 
limited, short of extreme findings.  
 
Conducting a trial assessing male fertility that uses pregnancy rate as an outcome is neither 
practical nor feasible.  Thus, the main outcome measures of a clinical trial assessing testicular 
toxicity in men are semen parameters.  This guidance provides information on the design and 
conduct of such a trial. 
 
Sponsors of anticancer drugs that fall under the scope of the International Conference on 
Harmonisation guidance for industry S9 Nonclinical Evaluation for Anticancer Pharmaceuticals 
should consult with the Office of Hematology and Oncology Products before initiating follow-up 
studies evaluating testicular toxicity.3 
 
 
III.  NONCLINICAL EVALUATION 
 
A.  Introduction 
 
Nonclinical evaluation of the male reproductive system is a standard component of the 
nonclinical safety assessment during drug development.  Evidence of adverse drug- 
related findings on the male reproductive system in animals, specifically the accumulating 
evidence in appropriate species, informs whether there is a need for an evaluation of testicular 
toxicity in men. Testicular toxicity is routinely assessed using:  
 
  Repeat-dose toxicology studies with 2 to 4 weeks of drug exposure in two species, unless 
only one species is studied based on pharmacological relevance 
                                                 
3 We update guidances periodically.  To make sure you have the most recent version of a guidance, check the FDA 
guidance page at https://www.fda.gov/RegulatoryInformation/Guidances/default.htm.  
2  Contains Nonbinding Recommendations 
 
  Assessment of male fertility in rodents (when applicable) 
 
Additional information may come from embryo/fetal reproductive and developmental toxicity 
studies and fertility assessment after prenatal, neonatal, or juvenile exposure.  Sponsors should 
also consider known class effects and/or potential target-related effects. 
 
B.  Nonclinical Study Design Considerations 
 
The sponsor should provide a rationale for the choice of doses, duration of exposure, and species 
used to investigate male reproductive toxicity in nonclinical studies.  All studies should include a 
control group of animals, and early safety evaluation programs should employ both a rodent and 
nonrodent species, based on pharmacological relevance.  The objective of early preclinical safety 
studies is to define pharmacological and toxicological effects using conventional approaches 
where immature animals are routinely used at study initiation.  Unless studies are intended to 
support dosing in pediatric patients, sponsors should consider the limitations of using sexually 
immature animals in subchronic toxicity studies because histology findings in immature animals 
may incorrectly suggest that fertility is impaired.4   
 
For chronic studies where nonhuman primates are the only relevant species based on 
pharmacology, the sponsor should ensure when assessing reproductive toxicity that male animals 
have attained sexual maturity by the end of the study.  However, this is not always feasible.  If 
impaired spermatogenesis is observed in maturing males, the sponsor may need to further 
investigate potential effects in fully mature males, on a case-by-case basis.  FDA encourages 
sponsors to discuss with the review division appropriately designed reproductive toxicity 
assessments in nonhuman primates. 
 
FDA considers histopathological evaluation of the reproductive organs of sexually mature 
animals to be an appropriately sensitive endpoint for evaluating testicular injury in animals.  
Toxicology studies should include an examination of the histopathology of the testes, seminal 
vesicle, epididymis, and prostate with appropriate fixation and staining of the testes.5 If adverse 
findings in gonadal tissues were observed in the repeat-dose toxicity studies, histopathology 
assessment of the reproductive tissues in the nonclinical male fertility study/studies may provide 
additional evidence for the human risk assessment.  The persistence versus the reversibility of 
adverse effects in a group sacrificed after a specified period of drug withdrawal (sufficient to 
                                                 
4 We support the principles of the 3Rs (reduce/refine/replace) for animal use in testing when feasible.  FDA 
encourages sponsors to consult with review divisions when considering a nonanimal testing method believed to be 
suitable, adequate, validated, and feasible.  FDA will consider if the alternative method could be assessed for 
equivalency to an animal test method. 
 
5 Creasy DM, Chapin RE, 2013, Male Reproductive System.  In: Haschek WM, Roosseaux CG, and Wallig MA, 
editors, Haschek and Rousseauxs Handbook of Toxicologic Pathology, 3rd ed., London: Academic Press, 2541
2542; Creasy D et.al., 2012, Proliferative and Nonproliferative Lesions of the Rat and Mouse Male Reproductive 
System, Toxicol Pathol, 40:40S121S; Chapin RE, 1988, Morphologic Evaluation of Seminiferous Epithelium of 
the Testis. In:  Lamb JC and Foster PMD, editors, Physiology and Toxicology of Male Reproduction, San Diego, 
California: Academic Press, 155178; Hess RA and Moore BJ, 1993, Histopathological Methods for Evaluation of 
the Testes.  In: Chapin RE and Heindel JJ, editors, Methods in Toxicology, Vol. 3, Pt. A. Male Reproductive 
Toxicology, San Diego:  Academic Press, 5285. 
3  Contains Nonbinding Recommendations 
allow reconstitution/recovery of spermatogenesis) on the reproductive system is an important 
consideration in the risk assessment. 
 
C.  Nonclinical Findings That Raise Concern for Male Fertility 
 
In general, reproductive toxicity findings in male animals that raise concern for impaired fertility 
include, but are not limited to, atrophy, degeneration, necrosis, or hypocellularity of testes; 
increased seminiferous tubule degeneration or necrosis; germ cell depletion; or other pathology 
that may suggest impaired reproductive function.  In addition, findings in other associated male 
reproductive organs (e.g., prostate, seminal vesicles, epididymis) may be suggestive of testicular 
toxicity.  The sponsor should consider clinical evaluation of testicular function for direct-acting 
testicular toxicants that are associated with decreased reproductive function and/or adverse 
histopathology. 
 
The significance of adverse findings in the toxicology and fertility studies increases if: 
 
  The incidence and/or severity of the findings increase with dose and/or duration of 
treatment 
 
  The reproductive findings occur in multiple species 
 
  The reproductive findings occur in tissues bilaterally  
 
  The adverse histopathology correlates with effects on reproductive organ weight 
 
  A finding does not resolve after a period of one or two spermatogenic cycles or after five 
half-lives following the last drug dose 
 
  The adverse findings occur at clinically relevant exposures 
 
  The adverse findings are seen at pharmacokinetic exposures that result in a safety margin 
comparable to clinical exposure 
 
Although histology is the most sensitive way to detect testicular and sperm quality toxicities, 
findings of reduced fertility, impaired mating behavior, and reduced capacity to mate in male 
fertility studies are concerns in and of themselves.  These findings are especially concerning if 
they are corroborated by histopathological evidence of adverse effects on reproductive tissues in 
repeat dose toxicity studies.  Findings that are suggestive of perturbations of the endocrine 
system are also a concern because changes in hormone homeostasis may adversely affect male 
(and female) reproductive physiology and performance.  For example, drug-induced alterations 
in endocrine function can affect testicular weight, gamete maturation and release, sperm count, 
and/or fertility. 
 
Table 1 summarizes findings in nonclinical studies that may increase the level of concern for 
impaired fertility. 
 
4  Contains Nonbinding Recommendations 
Table 1.  Nonclinical Findings That May Increase the Level of Concern for Impaired 
Fertility in Men 
 
General Nonclinical Findings to Consider in Male Fertility Risk Assessment 
Finding occurs at clinically relevant exposures or small multiples of the clinical exposure 
Finding occurs in multiple species 
Finding increases in incidence and severity with increasing duration of exposure 
Finding does not resolve, or at least show partial recovery, after one or two spermatogenic cycles or after five 
half-lives following the last drug dose 
Finding occurs bilaterally in paired organs  
Finding is rare in healthy untreated animals 
Reproductive organ weight change (increased or decreased weight) correlates with adverse histology 
Decreased male fertility and impaired mating behavior 
Sperm quality adversely affected (count, motility, or morphology) 
Signs of hormonal perturbation: 
  Anti-androgenic signs  decreased weight and maturation of male sexual organs, including seminal 
vesicles and ventral prostate when weighed with their secretions, clinical signs suggestive of reduced 
aggressiveness (e.g., lethargic or reduced mating behavior, feminization of males) 
  Androgenic signs  masculinization of females (decreased fertility, female sexual organ pathology, or 
estrus cyclicity), decreased testes size, and impaired spermatogenesis 
 
D.  Confounding Factors 
 
Numerous factors can confound apparent male reproductive toxicities.  The use of drugs that 
cause a reduction in body weight or impair neuromuscular/neurological function may result in 
signals consistent with impaired reproductive function.  When decreased spermatogenesis is 
detected in testicular histopathology examinations, it is important to document the reproductive 
age of the nonclinical model and to determine if the drug can have temporary or permanent 
effects on testicular development and spermatogenesis.  Drugs that cause adverse effects on 
sperm quality in rats, without an effect on mating outcome, may still represent a risk to human 
males because these findings may indicate undesirable effects on testicular function, independent 
of mating outcome.  For testicular toxicants where the mechanism of action is based on changes 
in hormone levels, the sponsor should clinically monitor hormones. 
 
E.  Follow-Up Investigations 
 
Based upon an assessment of the findings from the nonclinical toxicology studies and any 
additional findings, the sponsor should consider additional nonclinical studies to characterize an 
observed male reproductive toxicity on a case-by-case basis.  Follow-up studies could contain 
some of the following assessments: 
 
  A demonstration of the potential for recovery from the adverse finding after cessation of 
dosing, if not available from the initial toxicology studies 
 
  A reproductive hormone analysis, recognizing that hormone concentrations can vary 
significantly between animals and over the course of a day and over the course of the 
study 
 
  A determination of the target cell type (e.g., germ cell, Leydig cell, Sertoli cell) 
5  Contains Nonbinding Recommendations 
 
In selected cases, adding fertility and/or sperm quality analysis to repeat-dose toxicity or fertility 
studies may be appropriate.  The length of dosing in the premating period of the male fertility 
study could be increased to cover an entire spermatogenic cycle and epididymal transit (for 
example, approximately 63 days in rats) to determine the extent of expected or observed 
toxicities in previous studies.  A confirmatory study in a second species may be useful in cases 
where the finding is suspected to be species dependent (e.g., when effects are caused by a 
species-specific metabolite). 
 
FDA does not intend for these nonclinical discussions to be comprehensive but rather to serve as 
a starting point for evaluating the risk of testicular injury in humans.  
 
 
IV.  MONITORING OF THE TESTES DURING CLINICAL TRIALS 
 
Whether an evaluation of testicular toxicity in men is needed depends on various factors, 
including the mechanism of action of the testicular toxicity, if known, duration of therapeutic 
use, exposure multiples for the expected clinical exposure, intended target population, and 
indication of use.   
 
A plan to minimize and monitor for the risk of human testicular injury should be in place early in 
clinical development for drugs that have a potential to cause human testicular toxicity based on 
nonclinical findings at anticipated clinically relevant exposures, taking into account the other 
aforementioned factors.  The sponsor can discuss this plan with the appropriate review division 
as part of a pre-investigational new drug application (pre-IND) meeting or develop the plan and 
provide it with the original IND or at other time points in the IND phase, as appropriate.  The 
sponsor can also discuss these issues at milestone or other meetings.  
 
It is not possible to provide a single risk minimization and monitoring plan for all drugs with a 
potential for human testicular toxicity.  The sponsor should individualize each plan considering 
factors that may include 1) the type, severity, consistency, and reversibility of the findings in the 
toxicology studies; 2) the duration and dose of clinical exposure relative to the duration margin 
(i.e., if seen with short-term or only exposure in chronic toxicology studies) and exposure margin 
(minimum observed adverse effect level margins close to (i.e., less than tenfold) clinical 
exposures or with substantial margins to clinical exposure); and 3) consideration of benefit-risk 
based upon the intended indication (i.e., treatment of a serious/life-threatening disease with 
unmet medical need or a common disease without unmet medical need).  
 
Risk mitigation may include restricting the population to be studied, at least in the early phases 
of development, if appropriate, for the drug in question.  For example, the drug could be initially 
investigated only in females or vasectomized men who have completed family planning.  
Although initial use in females and these vasectomized men may not contribute clinical data 
relevant to semen quality, assessment of reproductive hormones could be informative.  This 
approach may allow initial pharmacokinetic, pharmacokinetic/pharmacodynamic, safety, and 
efficacy evaluations of the drug while the sponsor obtains additional nonclinical testicular safety 
6  Contains Nonbinding Recommendations 
data, and better understands the likely clinical exposure range (to assess the clinical exposure 
margin relative to the no observed adverse effect level).  
 
In circumstances where men will be exposed to the drug, the potential risk of testicular injury 
should be clearly conveyed in the informed consent.  During the clinical trial in these subjects, 
information should be gathered on the effect of the drug on the testes.  This information should 
be based on the specific circumstances of the subjects exposure and could include semen 
analyses, as follows: 
 
  At baseline 
 
  At one spermatogenic cycle (13 weeks) after starting the investigational drug 
 
  At one spermatogenic cycle (13 weeks) after drug discontinuation or following complete 
drug elimination, whichever is later, with an assessment for recovery from changes in 
semen parameters, if significant adverse changes are seen at the 13-week evaluation  
 
Subjects should abstain from ejaculating for at least 48 hours and a maximum of 7 days before 
each semen collection.  For each assessment time point, semen analysis should be based on the 
average of two semen specimens collected several days apart.  In addition, the sponsor could 
assess other biomarkers of testicular injury, such as serum concentrations of testosterone, follicle 
stimulating hormone (FSH), and luteinizing hormone (LH).  The sponsor should factor in the 
diurnal and episodic nature of hormone secretion in the sampling scheme when performing 
hormonal testing. 
 
 
V.  DESIGN OF A CLINICAL TRIAL TO EVALUATE THE EFFECT OF A DRUG 
ON THE TESTES 
 
Based on the nonclinical findings, the results from initial human testing, and the intended use of 
the drug being considered, it may be appropriate to conduct a dedicated clinical safety trial 
having as its primary purpose an evaluation of the effect of the drug on testicular function.  
Sponsors should discuss with the review division the appropriateness of conducting such a safety 
study before or in parallel with the phase 3 trials.  A separate safety trial may not be necessary 
under certain circumstances, such as if the drug belongs to a class with known effects on the 
testes (e.g., radiomimetics, androgens, anti-androgens).  The following section describes basic 
features that the sponsor should consider when designing such a trial. 
 
A.  Subject Selection 
 
Trial subjects should be men considered to have normal potential for fertility as reflected by 
semen parameters.  
 
7  Contains Nonbinding Recommendations 
We recommend that subjects have semen parameters that equal or exceed the generally accepted 
5th percentile values of the World Health Organization (WHO) reference values.  As of 2010, 
these 5th percentile values are:6 
 
  Semen volume  1.5 milliliters (mL)  
  Total sperm per ejaculate  39 million spermatozoa per ejaculate  
  Sperm concentration  15 million spermatozoa per mL  
  Sperm progressive motility  32 percent  
  Sperm morphology  4 percent normal forms using strict Tygerberg method 
 
These values should be equaled or exceeded in at least two semen specimens that are collected at 
least several days apart at baseline.  Subjects should abstain from ejaculating for at least 48 hours 
and a maximum of 7 days before each semen collection.  The sponsor should consider the 
average of the two specimens to be the baseline semen characteristics for each enrolled subject.  
To the extent feasible, subjects should be representative of the population for whom the drug is 
intended, with careful considerations for factors, such as disease severity and concomitant 
medications, that may confound the semen parameters.  
 
B.  Trial Design 
 
We recommend a randomized, double-blind, placebo-controlled, parallel-arm trial.  We 
recommend that the trial randomize approximately 200 men in a 1:1 ratio to receive either the 
investigational drug or placebo.  In general, this sample size has been found to be adequate for 
the purposes of estimating cumulative distribution curves and producing a 95 percent confidence 
interval width that is reasonably narrow for the primary endpoint.   
 
The investigational drug should be administered at a dose and frequency that is representative of 
its intended clinical use.  In general, for drugs intended for chronic use, the drug should be 
administered for at least two human spermatogenic cycles, which is 26 weeks.  Drugs indicated 
for short-term use or intermittent retreatment should be administered according to the maximum 
duration of intended use; sponsors may need to discuss the actual duration of investigational drug 
exposure with the review division. 
 
Sponsors should obtain semen analyses at baseline, at the end of the first 13 weeks, and again at 
the end of the 26-week dosing interval for chronically administered drugs.  For drugs intended 
for short-term use or intermittent retreatment, sponsors should perform these analyses at baseline 
and 13 weeks after administration of the investigational drug.  For drugs with long half-lives 
(weeks to months), we recommend the sponsor discuss with the review division the appropriate 
timing of semen analyses to adequately evaluate the drug effect.  Subjects should abstain from 
ejaculating at least 48 hours and a maximum of 7 days before each semen collection.  For each 
assessment time point, two semen samples should be collected several days apart.  The methods 
of collecting and handling of semen samples should be standardized for all sites in a trial.  A 
single central laboratory should process and analyze all semen samples for the purposes of 
                                                 
6 Cooper TG, Noonan E, von Eckardstein S, et al., 2010, World Health Organization Reference Values for Human 
Semen Characteristics, Hum Reprod Update, 15(3):231245. 
8  Contains Nonbinding Recommendations 
consistency and quality assurance.  If there are significant challenges with adhering to these 
standard approaches, the sponsor should discuss alternative plans with the review division.   
 
The primary endpoint of the trial should be the percentage of subjects in each group who 
experience a 50 percent or greater decline in sperm concentration, compared to baseline, 13 
weeks after starting the investigational drug (short-term use or intermittent retreatment drugs) or 
after 26 weeks of drug exposure (chronically administered drugs).  Currently, sperm 
concentration is considered the most reliably quantifiable semen parameter that has potential 
utility in providing information about male fertility.  It should be noted, however, that no single 
semen parameter can predict fertility potential and that the sponsor should consider all 
parameters in a semen analysis.  Therefore, the sponsor should evaluate as secondary endpoints 
changes from baseline in sperm concentration, ejaculate volume, total sperm per ejaculate, 
motility, and morphology.  The sponsor could also consider comparisons to WHO reference 
values and population-specific values as additional secondary endpoints.  The semen parameters 
at baseline and during treatment should represent the mean of two semen samples collected a few 
days apart at each time point. 
 
The sponsor should consider evaluating hormones, such as serum testosterone, FSH, and LH, in 
cases where changes in semen parameters are suspected to be related to hormonal perturbation.  
In addition, these hormonal evaluations may help to inform on the drugs effect on testicular 
function. 
 
Individual subjects who experience a 50 percent or greater decline in sperm concentration should 
be re-evaluated after at least a 13-week drug-free interval to assess the recovery following drug 
exposure.  An evaluation of recovery after a longer drug-free interval may be necessary for drugs 
with particularly long half-lives.  In these affected subjects, the mean of at least two semen 
analyses collected a few days apart at the end of the drug-free interval should be used to 
determine the change from baseline and change from the last on-treatment values of the semen 
parameters. 
 
C.  Presentation of Results 
 
The sponsor should base the primary analysis on all subjects who have baseline and at least one 
post baseline semen sample and should include a prespecified approach for handling missing 
data.  The proportion of subjects experiencing at least a 50 percent decrease in sperm 
concentration from baseline should be calculated together with the associated 95 percent 
confidence interval for the difference between the drug and placebo groups. 
 
In addition, the sponsor should construct a cumulative distribution plot for the primary endpoint 
for each treatment group.  The x-axis should display changes from baseline in sperm 
concentration ranging from 100 percent decrease (i.e., minus 100 percent or azoospermia) to the 
maximal observed increase.  The y-axis should display the proportion of subjects who 
experienced a percentage change in sperm concentration, at the primary time point, equal to or 
less than the corresponding x-axis value. 
 
9  Contains Nonbinding Recommendations 
Figure 1 shows a sample plot.  This plot shows that approximately 50 percent of subjects treated 
with either the investigational drug or placebo had a decrease in sperm concentration from 
baseline during treatment.  It also shows that a decrease in sperm concentration of greater than 
50 percent occurred in approximately 5 percent of the subjects who were treated with either the 
investigational drug or placebo. 
 
Figure 1.  Example of a Cumulative Distribution Plot 
 
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Percentage change from baseline  
Study arm:      Drug  +++ Placebo 
 

For each treatment group, the sponsor should calculate the median change from baseline in 
sperm concentration and in each secondary endpoint.  The associated 95 percent confidence 
interval for the difference between the drug and placebo groups should be shown for all 
endpoints. 
 
The percentage of subjects having individual secondary semen parameters within the normal 
reference range at the end of the treatment period should be presented for each treatment group.  
The percentage of subjects having all secondary semen endpoints within the normal reference 
range at the end of the treatment period should also be presented for each treatment group. 
 
We recommend that sponsors include tables showing shift analyses from baseline to week 13 (or 
26 for chronically administered drugs) for each of the primary and secondary endpoints for each 
treatment group.  Each table would include shift analyses from within the reference range at 
baseline to above the reference range at week 13 (or 26 for chronically administered drugs) and 
from within the reference range at baseline to below the reference range at these time points. 
 
The report should also include a discussion of reversibility of the findings during the drug-free 
follow-up period, if applicable. 
 
10  Contains Nonbinding Recommendations 
D.  Conclusions 
 
The sponsor should consider the potential for risk to humans when nonclinical studies 
demonstrate drug-related adverse effects on male reproductive organs, semen analysis, and/or 
fertility.  If assessment of testicular toxicity in men is indicated, plans to address and monitor for 
the risk of testicular injury should be in place early in clinical testing in male subjects or at the 
appropriate phase of drug development.  The primary purpose of the clinical semen safety trial is 
to evaluate human testicular function based on nonclinical findings of testicular toxicity that 
cause concern. For FDA, the semen safety trial does not directly evaluate drug effect on human 
male fertility, but the trial can provide useful information about drug effect on commonly used 
measures of testicular function.  
 
In general, it is not possible to stipulate firm guidelines for interpretation of these trial results 
and, a priori, specify results that would resolve the concern of testicular toxicity.  Sponsors 
should evaluate individually each drug, its intended use, and the results of a semen trial as 
outlined in this guidance.  Ultimately, the acceptability of the adverse effects of a drug on 
testicular function should be based on the overall benefit-risk assessment of the particular drug 
and indication being sought. 
 
 
11