Microdosing in Phase O: A Q&A with J. Scott Tarrant, Xceleron

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Pharmaceutical Technology, Pharmaceutical Technology-08-01-2008, Volume 2008 Supplement, Issue 3

J. Scott Tarrant, executive vice-president of Xceleron, explains the role of microdosing in drug development. He describes how microdose data can be used to predict pharmacological dose absorption, distribution, metabolism, and excretion/pharmacokinetic outcomes using accelerator mass spectrometry.

Accelerating drug development is crucial for the pharmaceutical industry. Microdosing is a promising strategy for determining the pharmacokinetic (PK) properties of a compound, including oral absorption, rate of metabolism, and excretion characteristics at the earliest point possible in drug development. J. Scott Tarrant, executive vice-president of Xceleron (Germantown, MD), a provider of predictive clinical research and coordinator of the European Union Microdose Accelerator Mass Spectrometry Partnership Program (EUMAPP), explains its use.


PharmTech » How is microdosing used in drug development?

»Tarrant: Microdosing, sometimes referred to as a human Phase 0 study, is the administration of subpharmacologic doses of experimental drugs to human volunteers, up to a maximum of 100 μg. For purposes of this discussion, I will only address studies conducted using accelerator mass spectrometry (AMS), but studies have also been conducted using positron emission tomography and, in some cases, liquid chromatography–tandem mass spectrometry (LC–MS/MS). The intent of the microdose study is to get a very early read on the PK of novel molecules to make critical decisions about which molecules to advance. This evaluation can be accomplished in as little as six months with minimal preclinical toxicology and GLP [good laboratory practices] material under an exploratory investigational new drug application (eIND) in the United States or under an abbreviated common technical document (CTD) in Europe (1, 2). A microdose study allows drug developers to rank the order of several drug candidates coming out of discovery and select those that are most appropriate based upon human PK data. In the traditional development cycle, this evaluation can take up to 18 months for one molecule at a cost of $3–5 million. In a Phase 0 study, however, one can test as many as five molecules in one clinical study and gain all of the relevant PK data in six months. Microdosing is about making decisions on early drug-candidates in the context of all the other tools at a company's disposal by using humans as the model instead of animals while at the same time de-risking the likelihood of clinical failure later.

PharmTech » What is EUMAPP?

»Tarrant: EUMAPP is funded by the European Commission under the Framework Program 6 (3). It was founded in 2006, and the project is coordinated by Xceleron. The group consists of 10 organizations from five different European countries (France, The Netherlands, Poland, Sweden, and the United Kingdom). Its goal is to boost Europe's expertise in microdosing and to show the role that AMS can play in enabling more effective drug development. The EUMAPP project tested seven different drugs to show the reliability of the microdosing approach for predicting a drug's PK at pharmacological doses. The group's aims are to certify AMS as the most accurate and appropriate technology for reproducible measurements required by microdosing studies and to develop an in silico modeling application to predict PK parameters from data derived from microdosing studies.

PharmTech » Can you highlight EUMAPP's recently released microdosing data?

»Tarrant: The details of the EUMAPP project have not yet been disclosed as scientific discussion is ongoing. The data will be published in a peer-reviewed journal as soon as all the EUMAPP participants are in agreement that all calculations are accurate.

Seven compounds were tested as part of EUMAPP: clarithromycin, fexofenadine, paracetamol (acetaminophen), phenobarbital, propafenone, sumatriptan, and "S-19812," an investigational compound by Servier (Neuilly-sur-Seine, France) that was dropped in Phase I development. These compounds were chosen to rigorously test the predictability of human microdosing by studying drugs that exhibited properties in humans that are difficult to predict in animal or in vitro models and drugs with properties that might be difficult to predict at a therapeutic dose from microdose data.

All seven drugs were tested at both a pharmacologic dose and at a microdose (one hundredth of the pharmacologic dose, not to exceed 100 μg). For six of the seven drugs, an intravenous (IV) microdose was compared with an IV therapeutic dose from data generated within EUMAPP and with the literature or from EUMAPP data alone. For all seven drugs, an oral microdose was compared with an oral therapeutic dose. For five of the seven drugs, the comparison was made from data generated from EUMAPP and the literature. For two drugs, the comparison was with the literature only.

PharmTech » Xceleron has conducted its own microdosing research. Can you share these results and results from the Consortium for Resourcing and Evaluating AMS Microdosing (CREAM) study?

»Tarrant: A total of 25 drugs (that we know of) have been tested at both a microdose and pharmacologic dose. This total consists of several compounds from the peer-reviewed literature and a number of molecules that are not yet in the public domain as they are either Xceleron research projects or sponsor studies that are confidential. Of the 25 drugs, 21 exhibited linear PK, and 4 drugs exhibited some nonlinearity. Thus, 84% of the drugs examined showed linear PK between a microdose and therapeutic dose.

Table I: Pharmacokinetic data of select drugs in microdosing.

Tables I and II are a compilation of the published data and the data that we are aware of that is not currently in the public domain. The data includes fexofenadine (an EUMAPP molecule) as data on this drug was published by a Japanese group, but the listing excludes the other six EUMAPP molecules. The drugs tested in the CREAM trial (4) are also listed.


Table II. Pharmacokinetic (PK) data for select drugs and drug types in microdosing.

PharmTech » How are microdosing studies conducted? Are there any limitations in the type of the active ingredient that can be tested?

»Tarrant: Microdosing studies are normally conducted as a two-way crossover design with an oral microdose, followed by an appropriate wash-out period, then the IV microdose. The number of human volunteers is usually between four and six subjects per molecule. It is most common to test multiple molecules in parallel groups of subjects within one human Phase 0 clinical study. A microdose is categorized as a hundredth of the pharmacologic dose (or predicted pharmacologic dose) and cannot exceed 100 μg. The dose contains only the active ingredient, which has a radiolabelled tag incorporated into the compound structure, most commonly 14 C. To date, microdosing has been conducted predominantly on small molecules. Xceleron's research and development team is currently researching ways to label biologics without altering the activity of the compound. A significant amount of work has been conducted on biologics in preclinical studies; however, there are no published data from microdose testing of biologics in humans as yet.

PharmTech » How is AMS used in microdosing and what properties of the instrument allow for testing at increased sensitivity? How does the technique differ from traditional mass spectroscopy (MS)? Aside from AMS, are other analytical testing methods used in to evaluate the microdose?

»Tarrant: AMS was developed in the mid-1970s as means of analyzing archaeological artifacts through radiocarbon (14 C) dating. Xceleron pioneered AMS in biomedical analysis with the launching of the company in 1997. AMS is the most sensitive analytical technique ever developed: AMS is 1 million times more sensitive than liquid-scintillation counting (LSC) and up to 100,000 times more sensitive than conventional MS. Rather than measuring radioactive-decay events as LSC does, AMS separates elemental isotopes on the basis of mass, charge, and potential-energy differences. By analyzing 14 C at attomole (10-18) to zeptomole (10-21) levels, AMS allows human radioactive dosing to be reduced to a level where regulatory approval for the use of ionizing radiation is no longer required. Therefore, a traditional radioactive human clinical study can be converted to a nonradioactive study from a regulatory perspective by reducing the radioactive exposure up to 1000-fold. This reduction permits safer human absorption, distribution, metabolism, and excretion to be investigated in a wider range of subjects. In theory, infants, women of child-bearing age, and patients could be included in the microdosing studie s.

AMS differs from MS in that AMS measures individual atoms and gives no structural information. MS quantifies different chemical structures. The individual atoms measured by AMS can be directly quantitated to the drug concentration in the sample by knowing the exact amount of radioactivity administered.

For microdosing, AMS is coupled with high-performance liquid chromatography (HPLC) as a separation technique. Total 14 C is measured first, which accounts for both the parent compound and metabolites. Using an HPLC-AMS method, the parent concentration is also measured. The difference between the two values measures the extent of metabolism and can provide valuable information as to whether the compound is significantly metabolized as it is absorbed, either in the gastrointestinal tract or by first pass through the liver.

There are certain circumstances where LC–MS/MS can be used in a microdose setting, depending on the drug's PK characteristics. More potent drugs may limit the application of LC–MS/MS because of sensitivity issues. Additionally, if an initial read on the extent of metabolism is important, LC–MS/MS will not be able to provide that information. The bottom line is that AMS is an important tool that can be applied to compliment other existing technologies including LSC, HPLC, and LC –MS/MS.

PharmTech » What predictive models are used?

»Tarrant: Microdosing data would always be used in conjunction with all other information collected on the molecule in question, whether it be in vitro, in silico, or animal models. From the standpoint of evaluating the data, it is no different from the process used in a typical Phase 1 study. The application of data will, of course, very much depend upon the specifics surrounding the development of the drug. For example, a microdose study on an anti-infective targeted at the lung was designed to evaluate whether the drug could be administered orally or parenterally through an inhalation route. Comparison between the intravenous and oral microdose data showed that the drug was very poorly bioavailable because of limited absorption and first-pass metabolism. Therefore, an inhalation route was necessary.

PharmTech » Can you explain the positions by the US Food and Drug Administration and the European Medicines Agency on microdosing? Is microdosing under consideration by the International Conference on Harmonization?

»Tarrant: In terms of toxicology requirements, in order to commence a microdosing study, FDA requires a single-dose, 14-day study in both sexes of an appropriate mammalian species with interim sacrifice and histopathology. The intended route of human administration should be used, and an 100-fold safety factor, taking into consideration allometric scaling for the species, should be used to select the dose for the clinical trial. FDA does not require any genetic toxicology or safety pharmacology to be performed (1).

EMEA requires a single-dose, 14-day study in both sexes of a justified mammalian species to include a control group. An interim sacrifice and histopathology are required. IV, as well as the intended clinical route of administration is recommended. (If the intended route is IV, then this route alone will suffice). Allometric scaling from animal species to man and using a safety factor of 1000 should be used to set the dose limit. For genotoxicity studies, an abridged Ames test and abridged human lymphocyte, chromosome aberrations, mouse lymphoma, or in vitro micronucleus test are sufficient (2).

For FDA and EMEA, the compound does not have to be GMP [good manufacturing practices] material but should come with a certificate of analysis. Ideally, the same batch used for human microdosing should also be used for the toxicology studies.

At the current time, I am not aware of any consideration to harmonize microdosing by ICH. It would seem a next logical step as the number of molecules tested this way continues to rise and the studies become more routine.

PharmTech » What are microdosing's limitations and where does the industry stand in adopting microdosing?

»Tarrant: The technique is most useful in candidate selection to rank the order of a lead compound against a number of backups. It is also useful in situations where there may be conflicting animal data and the addition of early human data may significantly aid the decision process. In addition, an IV microdose can provide data on the fundamental PK of a drug, not possible from an oral dose alone. A current limitation is that we have too few data to form general views on which compounds or classes of compounds the technique would be most useful.

Microdosing has been widely accepted by many companies as a way to get very early human data to increase confidence in taking a drug forward into further development. It is one method of killing drugs with inappropriate PK properties and only advancing the best drug candidates, thereby minimizing the risk of later-stage clinical failure. It is obvious that the industry needs to change given the dearth of new chemical entities being approved and the increasing number of late-stage failures.

In our experience, it has been the small-to medium-sized pharmaceutical and biotechnology companies that have been the major adopters of microdosing, but we are now starting to see a greater number of large pharmaceutical companies taking an interest in the technology. The decision process tends to be much quicker in a small company, and I believe that microdosing provides them with an edge whereby they can use scarce resources to really accelerate their programs by gaining early human data to enable smarter decision-making.

In my opinion, the continued use and advancement of microdosing in drug development will most likely require:

Continued support of the methodology by regulatory agencies through programs such as FDA's Critical Path Initiative

Increasing the amount of data in the public domain with comparative results between microdose and pharmacological dose PK across major classes of drugs. Xceleron coordinated the CREAM Trial in 2005–2006 and EUMAPP in 2007–2008 to do that. A similar public–private partnership in North America could lend further support to the value of microdosing.

Improvements in the process such as on-line separation techniques coupling HPLC and AMS. Currently the separation is conducted off-line.

Greater general acceptance of the technology by the industry. Xceleron's service offerings in Phase 1 with META-ID and IV-PK at pharmacologic doses and a microtracer of radioactivity have gained broad industry acceptance, and this, in turn, may bode well for both the technology and its application in microdosing.


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2. European Medicines Agency, "Position Paper on Nonclinical Safety Studies to Support Clinical Trials with a Single Microdose" (London, June 23, 2004).

3. EUMAPP, "European Union Microdose AMS Partnership Program (EUMAPP) Background Paper" (York, UK, Jan. 2006).

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