Regulatory Forum Opinion Piece

Abstract

In July 2015, the U.S. Food and Drug Administration (FDA) posted a new draft guidance entitled “Testicular Toxicity: Evaluation during Drug Development Guidance for Industry,” with a 90-day public comment period. As the nonclinical assessment of testicular toxicity often relies on the expert interpretation of pathology affecting the male reproductive tract, this draft guidance is considered directly relevant to the toxicologic pathology community. Therefore, a working group was formed through the Scientific and Regulatory Policy Committee of the Society of Toxicologic Pathologists (STPs) to provide a detailed review of the draft guidance. Specific comments on the guidance were submitted to the FDA by the STP. The draft guidance and all comments received are currently under review with the FDA. This commentary provides a summary of the components of the draft guidance and the comments submitted by the STP with acknowledgment of different perspectives reflected in comments from other respondents.

On behalf of the Society of Toxicological Pathology (STP), the Scientific and Regulatory Policy Committee drafted a response to Food and Drug Administration (FDA) Draft Guidance Testicular Toxicity: Evaluation during Drug Development Guidance for Industry, which was posted for comments in July 2015. The Executive Committee reviewed and approved the response, which was subsequently submitted to the Federal Register in October 2015. The Regulatory Forum of STP requested a summary of this response letter to inform Society members. The following represents a summary of the guidance and the STP response in the context of all responses submitted to the agency. All submissions, including the full STP response, may be viewed at https://www.federalregister.gov/articles/2015/07/17/2015-17557/testicular-toxicity-evaluation-during-drug-development-draft-guidance-for-industry-availability.

The stated purpose of the FDA guidance on evaluation of testicular toxicity is to assist sponsors who are developing drug products (human drugs and therapeutic biological products) that may have potential adverse effects on the testis based on nonclinical studies. The guidance, divided into 4 sections (and introduction), covers:

Difficulties evaluating testicular toxicity in humans.

Nonclinical evaluation including nonclinical study design, findings that raise concern for male fertility, confounding factors, and follow-up investigations.

Monitoring of the testes during clinical trials.

Design of a clinical trial to evaluate the effect of a drug on the testis including subject selection, trial design, presentation of results, and evaluation of results.

In review of comments submitted to the Federal Register in response to the draft guidance, a number of common themes surfaced. Those discussed below represent substantive (noneditorial) concerns expressed by multiple reviewers, supported in many cases by the literature and International Council on Harmonization (ICH) guidelines.

Overall, reviewers did not hold a unified view on the value of nonclinical end points discussed in the draft guidance. Some reviewers stated that the recommendation to consider hormone analysis, and/or the addition of fertility and/or sperm quality analysis, to select cases was unnecessary. Other reviewers proposed consideration of additional end points such as interim necropsies; the STP was the sole reviewer to suggest mammary histology as an additional end point. Although not clear from the responses, these differing opinions on study design may be due to past experience, class of molecules, cell type affected, and/or mechanism of action.

Nonclinical studies: The purpose of this section is best summarized in the concluding statements (guidance document section F, lines 193–200): “The (nonclinical) evaluation should consider the mechanism of action, route of exposure, duration of therapeutic use, exposure multiples for the expected clinical exposure, and indication of use, and should conclude with a determination of whether clinical assessment of semen parameters is recommended.” While the guidance does not directly associate adverse testicular histopathology with alterations in semen parameters, it does state that “clinical evaluation of testicular function should be considered only for direct-acting testicular toxicants, where decreased reproductive function is accompanied by adverse histopathology” (lines 163–65). The nonclinical studies section was the main focus of the STP response. Pertinent comments include:

FDA guidance is not fully harmonized with ICH guidance for animal use:

When discussing routine assessment of testicular toxicity, the FDA guidance describes repeat dose testing with at least 4 weeks of exposure as a component of the weight of evidence from animals. This appears to conflict with ICH guidelines that indicate studies of 2 to 4 weeks duration are sufficient to assess testicular toxicity (ICH 2005).

When discussing the need to use sexually mature models for assessment of fertility in repeat dose toxicity studies, the FDA guidance may result in exclusive use of mature macaques, including initial repeat dose toxicity studies, for biopharmaceuticals. ICH guidelines indicate that fertility in large animals should be assessed in repeat dose toxicity studies of at least 3 months duration using sexually mature animals (ICH 2011). This recommendation is corroborated by the recommendations of the STP (Lanning et al. 2002) which state that when dealing with known testicular toxicants or a compound on which fine testicular morphology is to be performed, sexually mature animals should be utilized. However, the use of sexually mature macaques in studies of less than 3 months duration is not discussed within ICH guidelines, and, historically, prepubertal to adolescent (2- to3-year-old) macaques have been used for initial toxicity testing.

As written, the FDA guidance may result in unnecessary nonclinical testing:

“When testicular toxicity is identified in repeat dose studies, additional evaluation may not be warranted based on class of drug (chemotherapeutic), drug indication (treatment of severe or life threatening conditions), intended patient population, relevant margin of exposure, and chronicity of intended use.” As written, the FDA guidance infers the presence of testicular toxicity within nonclinical studies uniformly triggers further investigation.

The guidance represents a potential change to current practice with the recommendation for “histopathology assessment of the reproductive tissues in the nonclinical male fertility study/studies … if adverse findings in gonadal tissues were observed in repeat-dose toxicity studies.” As this is not current practice, it is expected that this will potentially result in additional histopathology.

When discussing nonclinical findings associated with male fertility, the FDA guidance associates a number of adverse findings with heightened concern for testicular toxicity, although responses indicate that there are additional considerations, which are not described in the draft guidance, including:

Histopathological effects that correlate with organ weight changes: The STP stated that although adverse histopathology correlating with effects on reproductive weight usually indicates heightened concern, this statement is an oversimplification as stress has a confounding effect on organ weight and decreased body weight gain is a covariate for decreased organ weights. It is the STP position (Lanning et al. 2002) that absolute weights are generally more useful than relative values because testis, like brain, weight is less influenced by body weight than most other tissues.

Findings that do not resolve after one spermatogenic cycle: The authors agree that persistence is an important factor in determining adversity and directly contributes to assessment of risk. However, reversibility, persistence, and overall toxicity may be affected by the time to steady state, tissue distribution/accumulation, drug half-life (drug elimination), the regenerative capacity of the target tissue, the initial severity of the finding, and the species and strain demonstrating the toxicity. Multiple reviewers felt that one full spermatic cycle following drug dosing may not be sufficient or appropriate to assess resolution. Consistent with guidance (ICH 2012), the trend toward recovery may be more important than the dosing time frame based on variable washout periods and species-dependent recovery times. It was also noted by several responders, including the STP, that there can be a protracted period of arrested spermatogenic progression with some testicular toxicants such as cytotoxic chemotherapeutic agents, but that this does not preclude eventual reversibility of the insult. It is the STP’s position (Lanning et al. 2002) that it is not possible to say whether spermatogenic depletion is or is not reversible without carrying out an appropriate study.

Effects occurring across dose levels: It is unclear why the stipulation that adverse findings occurring across dose levels should be of inherently heightened concern, as the margin of clinical exposure should be considered. Additionally, throughout the guidance, the authors and reviewers note that safety margins are only relevant in the context of clinical benefit.

Findings that are suggestive of endocrine perturbation: The term “endocrine disruption” is not well defined within the context of testicular toxicity in the draft guidance, and reviewers held widely varying views of the association of endocrine perturbation and testicular toxicity.

Histopathological assessments do not necessarily correlate with alterations in reproductive function per se. In most nonclinical species, unless findings representing a specific toxicity (i.e., implicate an effect on germ or Sertoli cells) are widespread, impact behavior/libido fertility is unlikely to be affected.

Nonclinical findings that may increase the level of concern for infertility in men are further summarized in table format (C, Line 151). Multiple redundancies and discordances were noted between the text and the table. Of particular note, the draft guidelines do not differentiate between any findings within the testis and adverse findings, regardless of the severity or type. As a spectrum of background and nonadverse findings may occur, a definition of adversity in relation to testicular findings would be beneficial. Such a definition would assist regulators in the interpretation of testicular toxicity in nonclinical studies.

Monitoring of the testis during clinical studies: The guidance stressed early development (and discussion with the review division) of a plan to minimize and monitor the risk of human testicular injury. The draft guidance indicates that this plan should be in place early in the clinical development for drugs that have the potential to cause human testicular toxicity. Comments within this section of the guidance included:

Selection of clinical subjects: Reviewers felt that selection of subjects for clinical studies should not be based only on ability to measure fertility parameters in semen but rather should include vasectomized males as they could provide valuable hormonal data.

Semen analysis: Multiple reviewers questioned whether semen analysis would be voluntary and whether inclusion of analysis would be appropriate regardless of exposure duration.

Design of clinical trials: Trial design is based on the nonclinical findings, the results from initial human testing, and the intended use of the drug under consideration. It is somewhat unclear if a stand-alone clinical trial in male humans is required or if the recommendations described in the draft guidance could be applied to any clinical trial. Clarity regarding the timing (phase 1, 2, or 3) of investigation of potential testicular toxicity was requested from multiple reviewers. Further, clinical trials may not be warranted in cases of anticipated testicular toxicity based on drug class and mechanism of action (e.g., radiomimetics). Comments generated within this section of the guidance include:

Recommended percentage of sperm reduction in sperm count (50%) triggering follow-up evaluation was considered quite high by a number of reviewers. A smaller change may be relevant “on a clinical and population level.”

As with nonclinical trials, the utility of results after one spermatogenic cycle (13 weeks) was questioned, especially for compounds in which therapeutic levels are not quickly achieved, or exposures are sustained for a period after cessation of dosing (long half-life). Many of the known human testicular toxicants (oncology agents, radiation, occupational exposure) can take many cycles to recover, often up to 36 months.

Many inherent challenges with design, conduct, and interpretation of clinical trials evaluating testicular toxicity were discussed including the large N values necessary, the need for fresh samples to evaluate semen parameters, and the inherent variability within semen parameters. It was noted that semen analysis is not a test of fertility, but rather, a test of one aspect of testicular function. Multiple reviewers referred to the World Health Organization’s 2010 method for morphology assessment as the current standard (WHO 2010).

The STP noted that the main outcomes of clinical trials for assessment of testicular toxicity are centered on semen parameters, while the main end points of nonclinical studies are testicular histopathology. A correlation or association between these parameters would be useful to the readers.

Presentation and evaluation of results: This section did not receive comments and is best summarized by the final statement of the guidance (lines 376–78): “Ultimately, the acceptability of the adverse effects of a drug on testicular function should be based on the overall risk-benefit assessment of the particular drug and indication being sought.”

This editorial was meant to provide the readership of Toxicologic Pathology with an overview of key issues in the draft guidance on testicular toxicity; however, the STP and others provided additional specific comments not reviewed above, and these are recorded within the Federal Register for those with additional interest. The society awaits completion of the final FDA guidance and additional communication will follow at that time as necessary.