Clinical Pharmacology Considerations for Radiolabeled Mass Balance Studies

- [Kori] Good morning or good afternoon or good evening depending on wherever you may be.

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Welcome all to our Clinical Pharmacology Considerations for Radiolabeled Mass Balance Studies webinar.

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It is now my pleasure to introduce our presenters.

First we'll hear from Zhixia Grace Yan Danielsen, Deputy Director in the Division of Infectious Disease pharmacology, within the Office of Clinical Pharmacology, within the Office of Translational Sciences or OTS at CDER at FDA.

Then we'll hear from Anuradha Ramamoorthy, Master Scientist and Policy Lead in the Guidance and Policy Team, also within the Office of Clinical Pharmacology, within the Office of Translational Scientists or OTS at CDER at FDA.

Please join me in welcoming our first presenter, Dr. Danielsen.

- [Dr. Danielsen] Thank you for the calm introduction.

I'm delighted to kick up the discussion of the FDA Final Guidance on Clinical Pharmacology Considerations for Human Radiolabeled Mass Balance Studies.

I will start with brief background information, then explain why FDA is publishing this Guidance, and what is the scope of this Guidance.

After that, Dr. Ramamoorthy will join me to provide an overview of this Guidance.

First off, what is a human radiolabeled Mass Balance Study?

The study administered are carbon 14 or tritium radiolabeled investigational drug.

Following the administration, the radiolabeled drug and its metabolites are measured in blood, urine and feces.

Generally these studies are a direct way to obtain quantitative, comprehensive, and essential information of absorption, distribution, metabolism, and excretion, also called ADME of the drug in the body.

A Mass Balance Study offers crucial insights into the overall metabolism and its excretion pathways of a drug and it's a metabolize.

It helps identify which metabolites appear in plasma and their quantities relative to the administered drug.

As we advance the development of an investigational drug, it is important to consider several clinical pharmacology studies such as hepatic impairment, renal impairment, and potential drug drug interaction studies.

The findings from Mass Balance Studies are instrumental in determining whether studies for hepatic or renal impairment for certain DDI studies are necessary.

With this information, it can avoid unnecessary exclusion of patients with a varying renal or hepatic function in clinical trials.

Additionally, these studies provide input on which metabolites needs to be structurally characterized and tested for a nonclinical safety studies or DDI studies.

So why FDA is publishing this Guidance now?

Although Mass Balance Studies are routinely conducted, the FDA saw a specific need for a dedicated guidance on Mass Balance Studies to set expectations and recommend best practices for these studies, this will help ensure that the data provided in the new drug applications are consistent and of high quality.

By publishing this guidance, the agency wants to under score importance of using Mass Balance Studies to inform the overall drug development program.

The Final Mass Balance Guidance was published in July this year.

The Guidance provides the agency's recommendation regarding deciding whether or when to conduct a study, and considerations for design of study, and lastly, reporting study results.

Let's first look at the recommendations on the timing of the Mass Balance Study.

Generally, Mass Balance Studies are needed for all new molecular entities and should be conducted early in drug development, at the latest before initiating any late phase clinical trials.

As the information from Mass Balance Studies is leveraged to inform overall drug development program, it's a good practice to conduct the Mass Balance Studies early such that there's a sufficient information to justify the eligibility criteria in the safety and efficacy trials, and determine if additional safety testing for metabolize may be needed.

The Guidance also outline some scenarios where Mass Balance Studies may not be necessary.

For instance, if the finding from Mass Balance Studies are already available from acceptable literature sources, or included in FDA approved product labeling, then conducting another study might be redundant.

In addition, specific types of drugs might not require these studies.

For example, monoclonal antibodies, oligonucleotide therapeutics, and endogenous substances such as peptides and hormones typically do not need these studies if their metabolism and excretion pathways are well understood based on basic pharmacology and non-clinical ADME data,

unless structural modifications are expected to alter their ADME properties.

Additionally, drugs which exhibit minimal metabolism where the majority of the dose, 90% or more, is recovered in urine as the unchanges in parent drug may also bypass these requirements.

Lastly, drugs resulting in no or ineligible systemic exposure often do not mean Mass Balance Studies.

When the Mass Balance Study is not feasible, for example, due to safety concerns, the sponsors should use alternative approaches such as animal Mass Balance Studies, or non-radiolabeled clinical sample analysis, or in vitro assessments or a combination of these approaches to characterize the drug ADME problems.

Sponsor considering alternative approaches should consult with appropriate FDA review division.

This is the end of my presentation.

Now I will turn it over to Dr. Ramamoorthy who will continue to speak about guidance recommendations on study design and reporting results.

Thank you.

- [Dr. Ramamoorthy] Thank you Dr. Danielsen for the background and overview.

Moving on to some considerations for designing Mass Balance Studies.

Generally, Mass Balance Studies are non-randomized and open-label.

These studies can be conducted in healthy adult volunteers.

If safety concerns preclude enrolling healthy volunteers, then the study can be conducted in the intended patient population.

In general, having at least six evaluable volunteers is recommended to provide meaningful information.

Having fewer evaluable subjects may limit the interpretability of study results.

When determining the final number of subjects the sponsors should take into account any anticipated variability or known variability in pharmacokinetics including via polymorphisms in relevant pharmacology.

The dose of radioactivity should be estimated via dosimetry calculations based on data from animal studies.

Guidelines of other groups concerned with human safety should also be considered, as appropriate.

The position of the radio isotope should be in a chemically and metabolically stable position such that the parent drug in metabolites can be detected and quantified.

The dose of the non-radiolabeled investigational drug used in the Mass Balance Study should be the final intended dose if known, or at least in the anticipated therapeutic dose reach, taking into account the safety profile of the drug in the study population.

If the therapeutic range has not been identified at the time of conducting the Mass Balance Study, the study can be performed with a dose within the pharmacokinetic linearity range.

A single dose study is generally sufficient.

A multiple dose study can be considered when the investigational drug or active metabolite exhibit time dependent pharmacokinetics, or the study has to be conducted in patients and a single dose study is not feasible.

In such instances, bio analysis of non-radiolabel parent drug and metabolites of interest at study state can help interpret the study results.

The route of administration for the Mass Balance Study should include the final intended routes of administration unless precluded by practical consideration.

Example inhalation products.

The formulation used in Mass Balance Study is a fit for purpose formulation, which contains both radiolabeled and non-radiolabeled drug materials, and is different from the final intended formulation.

Although formulation differences may cause changes in ADME parameters, the formulation used in the study should still permit the collection of sufficient information that will fulfill the objectives of the Mass Balance Study.

As information on absolute bioavailability of the investigational drug can help interpret mass balance data and understand the overall drug elimination pathways, for orally administered drugs and an absolute bioavailability study can be combined with Mass Balance Study in a single protocol.

Preferably sample collection should continue until cumulative radioactivity exceeds 90% of the administered dose in urine and feces.

The potential causes for low recovery or large variability should be provided when the recovery is less than 90%.

Plasma, urine, feces, and other matrices as applicable should be collected for quantitative analysis of total radioactivity in parent drug and for metabolite profiling.

Ideally, sample collection should continue under the cumulative radioactivity exceeds 90% of the administered dose in urine and feces, and the radioactivity in urine and feces is less than one percent of the administered dose over a 24 hour period on two consecutive sample collection days.

For drugs with long half-life, parent or metabolite, when an extended stay in the clinic becomes impractical to achieve greater than 90% recovery, alternative sample collection strategy should be considered.

However, the estimated recovery based on interpolation and or extrapolation should be interpreted with caution because these estimations are typically based on the assumption that the drug excretion is under a first order process.

With respect to sample handling, plasma, urine, and feces samples should be properly stored and handled after sample collection and before analysis.

The stability of the investigational drug in active metabolites in corresponding matrices should be assessed to avoid misinterpretation of metabolite profiling results due to interference by degradation products.

For quantitative analysis of total radioactivity, parent drug and metabolites of interest, for example, active metabolite in plasma, urine, or feces, sample should be analyzed separately for each subject and pooling is not recommended.

Metabolite profiling is usually conducted after pooling sample in the matrix of interest, plasma, urine, or feces across time point for each subject or across subjects for each time point depending on the purpose of the analysis, and practical consideration.

Any pooing strategy should be described in detail in the study report.

In addition to the parent drug, metabolite profiling should be performed in plasma, urine, and feces sample.

The ratio of plasma metabolite to parent drug exposure can provide information on whether and which metabolite should be considered for further DDI evaluation.

Generally if a metabolite accounts for more than 10% of the total drug related exposure in plasma, urine, or feces, the metabolite should be structurally characterized.

Ideally, greater than 80% of radioactivity record in the excreta should be identified to assess the metabolic pathway of the parent drug.

If less than 80% is identified, the applicant should provide adequate justification.

With respect to bioanalysis, the choice of bioanalytical technique used and any associated method validation will depend on the objective of the Mass Balance Study.

Typically, both radiolabeled and non-radiolabeled analytical techniques are used.

For bioanalysis of radioactivity, the detection and quantification of radioactivity should be performed in all applicable biological matrices using radioactivity counting techniques, such as liquid scintillation counting, accelerator mass spectrometry, HPLC with radio detection, et cetera.

The quantification of unchanged parent drug and metabolites of interest should be performed in all applicable biological metrices using sensitive an analogical techniques such as LC-MS-MS. Validated bioanalytical method should be used for quantification of parent drug and metabolites of interest.

Moving on to reporting the study results.

The Mass Balance Study report should include the following.

Plasma and whole blood concentration versus time performance to total radioactivity, plasma concentration versus time profile for the parent drug and metabolites of interest, descriptive statistics of PK parameters for total radioactivity, parent drug and metabolite of interest in plasma, including AUC, Cmax, Tmax, terminal half-life,

the cumulative percentage of administered radioactivity dose recovered in the urine, feces, and total excreta versus time profile, quantitative information on the radioactivity associated with the parent drug and each identified metabolite in collected matrices, a biotransformation scheme with structures

or descriptions of the metabolites, if applicable.

The results are generally included in Section 12.3 Pharmacogenetics of the drug labeling.

This was a brief overview of the final Guidance.

With that let's move on to the CE question.

Results from a human Radiolabeled Mass Balance Study can, one, provide insight into the investigational drug's metabolism and excretion pathway, two, can inform the design of the drug development program, three, can help identify metabolites for which nonclinical safety assessments should be performed or four, all of the above.

The correct answer is four, all of the above.

Thank you for your time and attention.

I will now pass this over to the moderator for the Q and A session.

- [Kori] Thank you again to our presenters for the excellent presentations.

We will now begin our question and answer panel.

Joining us will also be Dr. Suresh Doddapaneni, Deputy Director of the Division of Inflammation and Immune Pharmacology within the Office of Clinical Pharmacology, or OCP, within the Office of Translational Scientists, or OTS, within CDER at FDA.

If you haven't had a chance to enter your question into the Q and A chat box, please do so now.

We will answer as many questions as time allows.

So for my first question, this question will be directed towards Dr. Doddapaneni.

What is your definition of late phase trial or pivotal trials?

- [Dr. Doddapaneni] Excellent question.

So in the Guidance we intentionally, you know, did not define precisely what this means, but adequate and well-controlled trials in phase three.

For example, you know, contribute significantly to the clinical, you know, pivotal clinical trial data, right?

So definitely before those trials.

Also, in some development programs in phase two also, you know, adequate and well controlled clinical trial data is, is also obtained, depending on the program and the objectives, and so on and so forth.

So any clinical trial where it is significantly contributing to the adequate and well-controlled database prior to that is what we are anticipating that the, that this study will be conducted.

Thank you.

- [Kori] Excellent, thank you for that information.

My next question is also for you Dr. Doddapaneni.

What route of administration is appropriate if practical considerations preclude the final intended routes of administration?

- [Dr. Doddapaneni] Another good question.

Ideally, studies should be conducted with the intended route of administration to maximize the utility of the information obtained.

However, if this is not feasible for practical reasons, an alternate route of administration should be used that will mimic as closely as possible with respect to ADME characteristics with the intended route of administration.

For example, inhale products for local effect in the lungs have absorption component from both lungs and GI tract.

In such a case, conducting the study via oral administration route is perfectly reasonable.

Thank you.

- [Kori] Thank you for that answer.

My next question will be for Dr. Ramamoorthy.

With respect to section five B, the final guidance document indicates that the population of the human Mass Balance Study should be healthy adult volunteers.

Routinely, these studies have been conducted in healthy male volunteers where possible, unless the drug is targeted for a female specific indication.

Is data in just healthy male subjects still acceptable to the agency in new drug application or NDA submissions or is there an expectation that female data should be generated routinely in the human ADME study?

- [Dr. Ramamoorthy] Thank you for that excellent question.

Typically in our experience there are specific inclusion and exclusion criteria for these Mass Balance Studies.

And in our experience, majority of these studies are conducted in healthy adult male volunteers.

But there may be instances where you can't study the particular drug in healthy adult male volunteer.

One of the reason could be the mechanism of action of the drug, which can then, or tie to the indication of the drug that may prevent some studies in healthy adult male volunteers, as well as there could be some sex specific adverse events based, and this may be known based on the animal Mass Balance Studies as well.

So in such instances these drugs may be studied in female volunteers.

And there's typically language again around the child bearing potential for these subjects.

Thank you.

- [Kori] Excellent, thank you for that response.

My next question is also for you Dr. Ramamoorthy.

Does the FDA also expect at least six completers in case the HMB study is conducted in oncology patients due to safety issues in healthy volunteers?

- [Dr. Ramamoorthy] Our general recommendation is to have six evaluable volunteers, patients, who have completed the study, and having fewer than six evaluable subjects or patients may limit the interpretability of the study results, especially if there is going to be variability in exposure.

And we also recommend that when selecting the final numbers that the sponsors take into account the anticipated or known variability in PK.

Thank you.

- [Kori] Excellent, thank you for that response.

My next questions are directed towards Dr. Yan Danielsen.

Can you define negligible systemic exposure for not performing a Mass Balance Study?

- [Dr. Danielsen] Sure, thank you for the question.

This usually apply to topically administered drugs such as cream or locally acting oral drugs, where the drug concentration are below detection limit.

Some often are highly variable sporadic and AOC cannot be reliable determinant.

In these cases, the Mass Balance Study will not be needed.

Thank you.

- [Kori] Excellent, thank you for that response.

My next question for you Dr. Danielsen.

What is the appropriate timing for the human Mass Balance Study, notably in oncology?

- [Dr. Danielsen] That's another excellent question.

So as I mentioned in presentation as the results from the human labeled Mass Balance Studies inform the overall drug development program, it is a good practice to conduct the Mass Balance Studies as early as possible before any late phase clinical trial begin, regardless of therapeutic areas.

Typically the Mass Balance Studies are conducted sometime between phase one and phase three, understanding that the development program or the timelines for anti-cancer agents are often faster than for other indications so, and so the sponsors should plan and conduct the Mass Balance Studies early so that the results can be back leveraged

to inform the subsequent clinical program, and reduce barriers to marketing authorization.

In some cases the Mass Balance Studies can be combined with phase one PK safety studies in cancer patients, or conducted before pivotal phase two studies.

Thank you.

- [Kori] Thank you for that response.

My next questions will be directed for Dr. Doddapaneni.

My question is how do you select the dose if the product is an extended release dosage form?

- [Dr. Doddapaneni] Good question.

So ideally the radio level dose should be the final intended dose or in the therapeutic dose range, or in the linear pharmacokinetics range.

If the therapeutic dose is unknown at the time of the Mass Balance Study.

This principle applies regardless of the dosage form.

In the case of an external release dosage form, practical and technical challenges preclude the use of a labeled external release formulation.

In this case, it is appropriate to use an immediate release dosage form with a dose that will give similar in vivo systemic exposure in terms of AUC and Cmax.

Thank you.

- [Kori] Excellent, thank you for that response.

My next question for you Dr. Doddapaneni.

Does the FDA agree that for drugs with non-linear PK, a multiple 14C dose study is the best approach to quantify parent drug and metabolites at or near study state?

- [Dr. Doddapaneni] Good question, so in this case, a multiple dose readable study is the best approach, you know, to get useful and relevant ADME information from the study.

Thank you.

- [Kori] Excellent, thank you.

My next question is for Dr. Ramamoorthy.

When would you expect a peptile ADME trial, or a peptide ADME trial?

- [Dr. Ramamoorthy] Thank you for the question.

Our recommendation is that generally for Mass Balance Study that they be performed for all new molecular entities as information obtained from the Mass Balance Study can help inform the subsequent drug development program.

However, there certain exceptions.

For example, peptide is one of those examples of exceptions for other drugs such as monoclonal antibodies, or oligonucleotides therapeutics, some of the endogenous substances like peptides and hormones, we know the metabolism and excretion pathway based on the basic pharmacology of the drug, the nonclinical ADME information, et cetera.

In such situations, a Mass Balance Study may not be needed.

However, if the structural modifications to these therapeutic modalities, including peptides, change the ADME properties of the drug, then as it modifies metabolism and excretion pathway, then a Mass Balance Study may be needed.

Thank you.

- [Kori] Thank you, my next question is also for you Dr. Ramamoorthy.

With respect to a combined protocol for hADME and absolute bioavailability trials, what is FDA's opinion on a timely separation of the two study parts by ergo six months between hADME part and absolute bioavailability part?

Does FDA prefer a second or separate protocol for the absolute bioavailability part?

- [Dr. Ramamoorthy] Thank you for the question.

This question possibly stems from the experience that more and more absolute bioavailability studies are being combined with the human radiolabeled Mass Balance Studies as two different parts.

In such instances we just ask for adequate washout period and that washout period may be very drug specific as well.

So the protocol for such a study would be both the part A and part B being included in such, in the protocol that is submitted to the agency.

And there are instances where these studies may not be combined as well, that the human ADME study may be separate from the absolute bioavailability studies as well.

So then two separate protocols would be submitted.

Thank you.

- [Kori] Excellent. Thank you.

And my last question for you, Dr. Ramamoorthy.

Do we need to collect blood samples for pharmacogenomic analysis?

- [Dr. Ramamoorthy] Again, an excellent question.

We recommend that the sponsors take into account their preliminary understanding of how the drug is metabolized.

If the drug is anticipated to be metabolized by drug metabolizing enzymes that are encoded by polymorphic pharmacogenes or they are transported by polymorphic transporters, then blood sample collection for pharmacogenomic analysis may be helpful.

And this will be especially helpful while, while interpreting the study results where there is PK pharmacokinetic variability.

And it would be hard to go back and retrospectively collect sample, then the, this would have to be considered in a separate study.

For those reasons where understanding how, having a preliminary understanding of how the drug is metabolized in transporter may help determine the need for a blood sample for pharmacogenomic analysis.

Thank you.

- [Kori] Excellent, thank you for that response.

My next question is for Dr. Danielsen.

The greater than 90% recovery is high.

What should one do if it is not met?

- [Dr. Danielsen] That's a good question.

So for tracking radio activity, the total recovery of radio activity in urine or feces should ideally exceed 90% of the administered dose.

We study high level of recovery because it allows for a reliable understanding of how the drug is eliminated from the body.

So if there is a lower recovery, we think it's a very important for the sponsor to investigate the potential reasons for this.

Whether it is variability in the ADME properties or analytical issues or other factors.

Identifying this causes is the key step to ensuring the reliability of the study results.

In addition, we don't see the Mass Balance Studies results in isolation.

When we review the results we always leverage information from other sources.

For example, in vitro studies, in vivo studies, clinical sample analysis so that we can get a full picture of the anatomy property of the drug.

Thank you.

- [Kori] Thank you, my next question for you, Dr. Danielsen.

You said that MB studies can be waived if urine excretion represents greater than 90% of unchanged parent drug.

How can this be determined unless a clinical MB study is performed?

- [Dr. Danielsen] Yeah, thanks for a question.

So the cases we often see is that if the animal studies or because the known drug class indicate that the the renal route will be the primary excretion route for the drug, oftentimes the sponsor will include the urine collection in the phase one SAD or MAD study.

In that case, the urine recovery can be determined in early phase one studies.

Thank you.

- [Kori] Excellent, thank you.

And my last question for you Dr. Danielsen.

Can we propose new methodologies not listed in the Guidance?

- [Dr. Danielsen] That's a good question.

The answer will be absolutely.

So the alternative approaches listed in the Guidance are not exhaustive, so the new methodologies can be proposed as appropriate, and as we know the technologies is evolving, we continue monitoring the trend in the conduct of the Mass Balance Studies, including the use of non-real label novel technologies to assess drug ADME.

Sponsor should consult the appropriate FDA review division when new methodologies are proposed for feedback.

Thank you.

- [Kori] Excellent, thank you.

My next question is for Dr. Doddapaneni.

Do we need to include the results from human Mass Balance Study in the end of the phase two package?

- [Dr. Doddapaneni] So end of phase two study meeting is a milestone meeting where all acquired data, clinical pharmacology and clinical data is summarized, and then, you know, the proposal for, you know, phase three trials, the study design, et cetera, is discussed.

So this is a good, good point where all the acquired data is summarized.

And you know, any data that needs to be acquired.

A plan for that is generally discussed at this meeting.

So any available human might as well instead data is a, you know, at this point summarizing the data as how it fits into the overall scheme of the drug development program is, is a good practice.

Thank you.

- [Kori] Excellent, thank you very much.

My next question for you Dr. Doddapaneni.

Do metabolites need to be quantified and characterized in both plasma and whole blood?

- [Dr. Doddapaneni] So in whole blood, generally the total radioactivity is quantified.

That's generally the practice.

It's only in plasma where the metabolites are characterized and quantified.

So whole blood it is just the total radioactivity.

Thank you.

- [Kori] Excellent, and my final question for you Dr. Doddapaneni.

When is a multiple dose study appropriate?

- [Dr. Doddapaneni] Good question, a single dose Mass Balance Study is the most simple and typical study design used to get the ADME data that is needed.

However, a multiple dose study may be considered if the drug or its active metabolites have time dependent pharmacokinetics as I discussed in response to an earlier question, or if a single dose study is not feasible.

For instance, if the study has to be conducted in patients.

In such a study, non-real level drug is administered until steady state is achieved and it's steady state, a single dose radio level drug is administered.

Thank you.

- [Kori] Excellent, my next questions are for Dr. Ramamoorthy.

For ADCs, are Mass Balance Studies necessary as payload is minuscule in the ADC moiety?

- [Dr. Ramamoorthy] That's a very good question.

We talked about the need for Mass Balance Study for new molecular entities and if the, the metabolism and elimination pathways on this well characterized for some of the other modalities, then a study may not be needed unless there is changes to the drug itself.

Particularly for ADCs.

I'd also refer you to the ADC Guidance, Anti Clinical Pharmacology Consideration for Antibody Drug Conjugate Guidance.

The Guidance recommends that, do, mentions that though Mass Balance Studies may not be feasible for antibody drug conjugates, efforts to assess, predict, or predict human elimination pathway of the payload should be considered and it can be through mechanisms

such as excreted, evaluating excreted metabolites in urine and feces in the early clinical trials, or through information from the animal studies, or in vitro studies of the payload.

Thank you.

- [Kori] Thank you, my next question for you, Dr. Ramamoorthy.

Should be avoided being tested in HVs. For instance, could a single dose of an epigenetic modifier could be unsafe for HV?

- [Dr. Ramamoorthy] Thank you for the question.

I assume to mean that if the drug is not safe for healthy volunteers, then can that be tested in patients?

It probably, that's the intent of the question.

We, that is something that if the study, if the drug is not safe for studying in healthy human volunteers, then this drug can be studied in patient population, that the intended patient population.

So we do recommend that it, the drug, especially if it's a epigenetic modifier that does not safe for a healthy volunteer study, be studied in patient population in order to understand the metabolism and elimination pathway of the drug.

Thank you.

- [Kori] Thank you, and my final question for you Dr. Ramamoorthy.

What if I don't know the final dose of the drug that will be used in phase three, and approved at the time of conducting the MB study?

- [Dr. Ramamoorthy] Thank you for the question, and that's again a very good question.

Just to step back, when the dose used in the Mass Balance Study is not the final approved dose and the clearance of the drug is not linear, this may then limit the interpretability of the Mass Balance Study results.

So generally we recommend that the Mass Balance Study use a dose of the investigational drug that is intended to be the final intended dose that would be used in the large scale studies, and would subsequently be included in the drug labeling.

If the final intended dose is not known, then the dose used in the Mass Balance Study should at least be in the anticipated therapeutic dose range and that can take into account the safety profile of the drug in the study population.

And if the therapeutic range has not been identified at the time of conducting the Mass Balance Study, then the Mass Balance Study should use a dose that's within the pharmacokinetic linearity range.

Thank you.

- [Kori] Excellent, thank you.

My next question is for Dr. Danielsen.

If organ impairment studies are conducted, can human Mass Balance Study be exempted?

- [Dr. Danielsen] Thank you for the good question.

So my short answer will be that depends.

As mentioned in the presentation, the Mass Balance Study not only help determine the overall metabolism and excretion pathways, which informs whether organ impairment study is needed or not and how.

It also helped provide a full picture of a metabolize that could contribute to efficacy, toxicity, and DDI, although in vitro or in vivo studies can provide useful information on metabolism without the use of renal label in the study, it's very difficult to make definitive conclusions on all circulating metabolize,

and that's not possible to fully assess the relevance of the non-clinical program as a part of the risk assessment for the drug product.

For example, if the drug is primarily renally excreted based on phase one studies and the drug has limited or minimum metabolism, then a renal impairment study is sufficient, the Mass Balance Study may not be necessary.

However, if the drug is extensively metabolized, a Mass Balance Study may be needed to fully capitalize the metabolic profile.

Thank you.

- [Kori] Excellent, thank you.

My next question for you Dr. Danielsen.

What are the study design considerations for long half-life drugs?

- [Dr. Danielsen] Yeah, that's a very good question.

So long half-life drugs does pose challenges for conducting Mass Balance Studies.

When an extended stay in the clinic becomes impractical to achieve high recovery, alternative sample collection strategies can be considered.

This may include strategies such as partial collection of samples, or collections of urine or feces in the outpatient setting.

So for example, in some Mass Balance Studies for long heart like drugs, we have seen the, this kind of a study design where the subject's first day over a fixed in, in clinic period, for example, approximately two half-lives and after discharge the subjects can return

at designated intervals such as weekly for 24 hour, or 48 hour in-clinic sample collections.

Then the recovery will be estimated based on interpolation of the recovery between the each visit.

In addition, we have seen some analytical tools such as accelerator mass spectrometry, AMS, have been used in some studies.

And this allows for low root activity dose in the range of nano curate to be administered to humans which alleviate the concern of extended exposure to root activity in humans for drugs with long half-life.

Thank you.

- [Kori] Excellent, thank you.

My next question will be directed towards Dr. Doddapaneni.

Can you explain if MB study is wavered for any pediatric medications, otherwise please advise how this study can be implemented for minors?

- [Dr. Doddapaneni] Good question.

So a drug that is predominantly developed for using pediatric medications, but drug still needs to be used in a safe and effective manner, taking into consideration on any pharmacokinetic changes due to extrinsic and intrinsic factors.

You know, for example, specifically any Pk pharmacokinetic changes related to drug related interactions, so from that perspective, information is still needed just as it would for a medication developed for adults.

But in this case, you know, the study can be conducted in adults.

That is perfectly fine because we are trying to assess the behavior of the, of the drug.

So in cases like this where there are these questions are very specific for your product, you know, reaching out to the review divisions, you know, that deal with that particular medication would be helpful to get feedback.

Thank you.

- [Kori] Thank you.

My next question for you Dr. Doddapaneni.

Are there ways to assess distributed tissue compartments such as CNS, adipose, or heart concentrations in these studies?

- [Dr. Doddapaneni] So, this particular Guidance is limited to studies conducted in humans.

And the way these studies are designed, the only matrices that are sampled are plasma, urine and feces.

As such, these studies do not provide information on distribution of the drug in tissue compartments such as heart, as they do in preclinical studies.

Thank you.

- [Kori] Excellent, thank you.

My next questions are for you Dr. Ramamoorthy.

My first question, why might a different formulation be needed for Mass Balance Studies?

- [Dr. Ramamoorthy] Thank you for the question.

The formulation that is typically used in a Mass Balance Study contains both radio labeled and a non-real labeled drug material.

And it's typically a fit for purpose formulation that's not the same as the final intended formulation because of this mixture of radio label, non-radio label drug material.

The formulation changes may cause some change in the ADME parameters, for example absorption.

But the formulation that's used in the Mass Balance Study should still allow for collection of sufficient information that fulfills the objective of the Mass Balance Study.

Thank you.

- [Kori] Excellent, thank you very much.

My next question for you, Dr. Ramamoorthy will be, one of the slides mentioned by you not suggested to pool feces samples for Mass Balance Studies if I hear correctly.

What is the reason?

- [Dr. Ramamoorthy] Thank you for the question.

Maybe I can repeat what I said that in the hopes that clarifies what the expectations are as laid out as the recommendations in the Guidance.

For the quantitative analysis of a total radioactivity and for parent drug and metabolites of interest, we recommend that the plasma, urine, feces samples should be analyzed separately for each of the subjects without pooling.

For metabolite profiling, it's typically conducted after pooling samples in matrices of interest, such as plasma, urine, or feces across time points for each subject or across subjects for each time point.

And this would depend upon the purpose of the analysis, practical consideration, the objective of the study, et cetera.

So if there is a pooling strategy that is used that should be described in detail in the clinical study report that's submitted for this particular study.

Thank you.

- [Kori] Thank you for those answers there.

My next question is for Dr. Danielsen.

Are Mass Balance Study results sufficient in order to propose dosing adjustments in renally or hepatically impaired patients for drug labeling, or are studies in such impaired patients also necessary?

- [Dr. Danielsen] Thank you for the question.

So the Mass Balance Studies can provide helpful information in terms of where the drug is eliminated, and that information can inform whether or not conduct certain organ impairment study.

However, it doesn't propose the dosing adjustment.

So based on the specific route of elimination, the organ impairment study still be needed to derive the dose adjustment in those patients.

Thank you.

- [Kori] Excellent. Thank you.

And my next question for you, Dr. Danielsen.

Pharma companies are increasingly submitting metabolite profiling and semi quantification using LC-MS or MS with high resolution MS to also provide structural identification when incorporating plasma samples from toxicology species.

This information provides insightful understanding of metabol, greater than 10% of total circulating material.

DDI risk and quantification of metabolite safety.

Do you see this meeting the intent previously achieved with Mass Balance Studies?

- [Dr. Danielsen] That's excellent question.

As we all know that LC-MS require a standard, a theme size standard to quantify the metabolite because the analyze response is structure or matrix dependent and subject to unpredictable ion suppression effects.

By contrast, quantification using radiometric technologies is a structure independent and less subject to matrix spec.

So that's a reason to use real label in a Mass Balance Study to get a full picture of the metabolite, circulating metabolite without using the standard.

Although the metabolic profiling using pharma clinical samples using LC-MS can be very helpful.

But as I mentioned, it can provide a full picture unless the metabolism of the drug is very simple, and by comparing the preclinical and the clinical samples, the metabolize or there's no unique metabolite, it may be doable using the LC-MS. But again, it is based on how complex

the metabolism of the drug.

Thank you.

- [Kori] Excellent. Thank you.

And that looks like that is all the questions we have time for.

Thank you again to our presenters for all the excellent information.

If you would like more information after today's webinar, we do have a few FDA resources for your review.

If you have any additional questions or comments, you can contact the SBIA a team at cdersbia@fda.hhs.gov.

And a few final reminders as we close out our day.

Recordings will be posted on our website within 10 business days at www.fda.gov/cdersbia.

Again, note that the CE claim code is available for two weeks only, and that deadline for this session will be November 26th.

Thank you again to all who participated today.

This concludes our webinar session, and we hope that you have a good rest of your day.