Human radiolabeling studies

Ideally, definitive CYP reaction phenotyping should be available before the initiation of clinical development. Unfortunately, more accurate data can only be obtained once clearance pathways are identified in human subjects, and human radiolabeled studies are generally not conducted as the first set of clinical studies for NCEs. In this context, CYP reaction phenotyping, performed using various human in vitro systems is expected to be as complete as possible (Bjornsson, 2003). 

Human radiolabel studies Radio-HPLC requires at least 200-1000dpm for detection of a metabolite (LumaPlate/ TopCount) sensitivity may be vastly increased by use of AMS Determined by molecular weight and dose carbon-14 preferred Metabolically stable remains in metabolite(s) of interest >98% >98% Determined by regulatory and ethical limits on radiation dose to subjects... 

Validation of the Model. The Corley model was validated using chloroform data sets from oral (Brown et al. 1974a) and intraperitoneal (Ilett et al. 1973) routes of administration and from human pharmacokinetic studies (Fry et al. 1972). Metabolic rate constants obtained from the gas-uptake experiments were validated by modeling the disposition of radiolabeled chloroform in mice and rats following inhalation of chloroform at much lower doses. For the oral data set, the model accurately predicted the total amounts of chloroform metabolized for both rats and mice. 

Using data from a human radiolabel ADME study in which metabolic pathways are fully delineated, in combination with in vitro data that identifies which enzymes are responsible for which metabolic pathways, values of can be estimated. 

Perform radiolabeled human ADME study as early as possible to define a major circulating metabolite. 

In terms of facilitating radiolabeled safety studies, AMS has generated considerable interest in the field of human microdosing studies, which are designed to generate human pharmacokinetic data earlier in the dmg discovery/development process... 

All the techniques described in this chapter contribute to improved metabolite characterization. Notably, these techniques will enable the generation of extended time profiles for plasma radioactivity, the assessment of free metabolite concentrations following plasma dialysis, and the characterization of metabolites at very low levels of exposure, for example, following inhaled or dermal routes of administration. Furthermore, the use of high-sensitivity techniques could impact the design of radiolabel studies, for example, by enabling a lower radiolabel dose to be administered to human subjects without compromising the ability to characterize the metabolic fate of the compound of interest. 

Roffey, S. J., Obach, R. S Gedge, J. I., Smith, D. A. What Is the Objective of the Mass Balance Study A Retrospective Analysis of Data in Animal and Human Excretion Studies Employing Radiolabeled Drugs. Drug Metab. Rev. 2007, 39, 17—43. 

If human metabolism studies with radiolabeled drug have not been performed prior to the conduct of reaction phenotyping studies, the initial experiments should use as complete an in vitro test system as possible, depending on the drug (e.g., tissue homogenates, liver slices, hepatocytes, etc.). 

Additionally, before the first study with radiolabeled test substance in man can be started, a risk assessment of a human radiokinetic study is mandatory. The estimation of the radiation exposure in humans given a radiolabeled dose is based on exposure data obtained typically from QWBA studies in animals. 

The aim of this kind of study is to characterize the Absorption, Distribution, Metabolism and Elimination of the investigational product in humans (hADME study), following an administration of the compound in a radiolabeled form. The use of a radiolabel allows identifying metabolites, which were not known beforehand, and to characterize them. In addition, using a radiolabel is - in most cases - the only way to establish a complete balance of the drug and its metabolites, which is required to validate the completeness and predicitvity of the results. 

The plasma exposure of a parent drug and major metabolites in the main toxicological species along with any related interspecies differences are of special interest for interpretation of toxicity studies. It is, however, only the data from the human ADME study with radiolabeled material that allows the final establishment of safety margins for all the relevant metabolites, as only then the systemic availability of all metabolites formed in human, is then known. Special attention has to be drawn to major human metabolites, which account for a considerable amount of the exposure (AUC) relative to the parent drug, and human-specific metabolites (Baillie [26]). For the human-specific metabolites and major metabolites that do not reach comparable systemic exposure in at least one animal species used in the different toxicity studies, separate toxicity studies should also be considered [26]. 

A human ADME study conducted using radiolabeled form of the NCE (usually referred to as 14C-human ADME) is a study that in most cases is required prior to start of the Phase III clinical trials and the data from the human ADME study are important components of the nonclinical sections of the NDA submission and drug label. The objectives of 14C-human ADME studies are to determine the total disposition of the NCE, encompassing mass balance, routes of NCE/metabolite elimination, and biotransformation pathways. 

The majority of the 14C-human ADME studies are conducted with a small number of healthy adult subjects (often between 6-8) and if bile collection is needed, a small group of additional subjects are included [228], Traditionally, due to ethical reasons, male subjects are selected for the 14C-ADME studies. Before the start of the 14C-ADME studies, study sponsors have the responsibility to determine stability of the radiolabel, purity of the radiolabel (distinguishing degradants from metabolites is very important), and conduct tissue distribution studies in nonclinical species preferably using quantitative whole-body autoradiography (QWBA) to detect radioactivity in tissues, organs, and excreta to determine the safe radioactivity dose. Nonclinical tissue distribution study data are extrapolated and used to show that radioactivity exposure of a specific tissue/organ will be well below the allowable limits to humans [229,230], Most of the 14C-human ADME studies consider a total radioactivity dose of 100[tCi or less to be safe [231],... 

Roffey, S.J. et al., What is the objective of the mass balance study A retrospective analysis of data in animal and human excretion studies employing radiolabeled drugs, Drug Metab. Rev., 39(1), 17, 2007. 

RDX has been detected in the serum, urine, and feces of one child who consumed unknown levels of RDX in the form of C-4 (91 % RDX). RDX was measured in the serum for 120 hours and in the feces for 144 hours after the presumed time of ingestion (Woody et al. 1986). The metabolities of RDX have only been found in animals by using a radiolabel ( C) (Schneider et al. 1977). Although this study found the radiolabel in the breath, urine, and feces, the chemical identity of the metabolites was not described. Therefore, metabolites cannot currently be used as biomarkers. In the one available human case study, RDX was found in the body following a single exposure, but no data are available regarding intermediate or chronic exposures. 

A major use of radiolabeled drugs in the in vivo situation is for metabolic profiling. These studies can be utilized to investigate interspedes comparisons of metabolic profiles in plasma urine and feces (or bile) in conjunction with toxicology or carcinogenicity studies or for performing definitive metabolite profiling in human ADME studies. 

The following section will focus on the detailed ADME studies conducted during the drug development process to help understand the disposition of a drug in nonclinical species and in humans. Since these studies are typically conducted with radiolabeled compound, a short section on the use of radiolabeled compound is included here along with a section on tissue distribution studies conducted to support the human ADME study. 

For the purposes of definition, nonradiolabeled reaction phenotyping is that conducted ahead of data from radiolabel studies in humans, and is typically conducted before first dosing of humans (Williams et ah, 2003). This differs from reaction phenotyping later in development, which in the presence of radiolabel or metabolite standards is more definitive in nature (Bjornsson et ah,... 

Experimental methodologies utilizing radiolabeled compound are similar to those where authentic standard is available (see Section 15.6.3). Radiolabeled studies can be conducted using the following expressed enzyme and/or human liver microsomes for the evaluation of A m-V max chemical inhibition, and/or correlation analysis using a characterized bank of human liver microsomes (Tuvesson et al., 2005 Zhu et al., 2005 Zhang et al., 2007). 

When projecting the drug-drug interaction potential of a particular drug, the extent to which the drug is eliminated by the inhibited enzyme is key to understanding the overall impact of the inhibition. Once the fraction metabolized (% dose) for the major metabolic pathways of a compound have been estimated (see Section 15.7.2) and the enzymes responsible for those pathways have been identified and the contribution quantitated, the overall fraction metabolized by a specific enzyme can be estimated (/m(enz))- Prior to the availability of the radiolabeled human ADME study,/m(enz) can be projected as a product of the predictions for overall fraction metabolized, fraction metabolized via a specific enzyme system, and fraction metabolized by the specific enzyme. It... 

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