Defining improved efficiency

The use of 14C tracers – and ultra-low level analysis – is the future of effective drug development, says Michael J. Butler.

What is interesting about the use of ¹⁴C tracers in drug development?
Michael J. Butler. Taking a fit-for-purpose approach using a ¹⁴C tracer in combination with ultra-sensitive accelerator mass spectrometry (AMS) can benefit contemporary efforts to make early drug development more effective and less costly. We've found that the ¹⁴C tracer/AMS combination can provide unique insight in Phase 0 (microdose), Phase 1 and Phase 2 clinical studies when combined with rigorous sample preparation.

Typically small molecule drugs have been labeled with ¹⁴C for the purpose of tracking (tracing) closely those events that could not be followed as easily without the tracer. For this to work, the amount of ¹⁴C tracer added must be such that it does not perturb the behavior of the larger (non-labeled) pool being studied or traced.

An example of a traditional study using a ¹⁴C tracer is the so-called human mass balance study performed at a pharmacological dose level. Such studies form part of regulatory submissions. This type of study typically employs a dose of 100 µCi, 3.7 MBq of radioactivity. Such levels can be measured by scintillation counting, a traditional analytical technique.

How does the traditional use of ¹⁴C tracer differ from what you do at Xceleron?
MJB. The biggest difference is derived from the specificity and sensitivity we can achieve with the 14C tracer/AMS combination we use. AMS measures isotope ratios in the femto- to attogram per mL range. This is many orders of magnitude more sensitive than traditional methods used to measure ¹⁴C or indeed methods such as LC-MS/MS used to measure un-labeled drugs.

What benefits does the ¹⁴C tracer/AMS combination confer on drug development?
MJB. Uses have changed with time as our customers have become comfortable with the technique. However, the the¹⁴C tracer/AMS combination is entirely complimentary and synergistic to traditional techniques such as scintillation counting and LC-MS/MS.

The earliest use our customers made of the ¹⁴C tracer/AMS combination was in the conduct of human regulatory mass balance studies but using ¹⁴C tracer at significantly lower specific activity than traditionally. We have conducted such studies at 100-200 nCi (3.7-7.4 kBq) radioactivity dosed. or 1000 lower than the levels described above. This approach has been taken to overcome inherent molecular instability resulting from radioactive decay. We have also used the sensitivity of AMS to measure later time-points, of several days in studies of long half-life drugs when the levels of drug drop below the detection limits of traditional analytical methods. We have been conducting these types of studies since Xceleron was formed 12 years ago. As far as we know, there are seven drugs on the market which were approved using Xceleron data from such studies.

Another common use of the ¹⁴C tracer/AMS combination is in the conduct of Phase 0 microdose studies. Our customers tell us that they are particularly valuable to provide very early go- no-go decisions, particularly for anti-infectives where pharmacokinetics can be pivotal for the drug's action. Most recently, we see demand from customers to use Phase 0 studies for purposes other than straightforward investigations of systemic PK. For example, we have conducted Phase 0 drug-drug interaction studies. We have also investigated drug transformations in isolated cell populations and drug disposition in CSF, skin blisters and other compartments. Xceleron has conducted Phase 0 microdose studies regularly for 10 to 12 years.

The area of greatest new interest in the use of the¹⁴C tracer/AMS combination is in clinical Phase 1. In particular, our clients are interested in getting insight to metabolite safety and fundamental kinetics such as absolute bioavailability. We have conducted studies in this area over the years but interest has grown significantly in the past year or so.

Why so much interest in the use of the¹⁴C tracer/AMS combination in clinical Phase 1 studies?
MJB. From what our customers tell us, it comes down to two things: Most importantly, the the¹⁴C tracer/AMS combination facilitates a clinical study design which is optimally suited to the issue under investigation; secondly, such studies can be conducted at a small financial increment to the funds already committed to the clinical Phase 1 investigation.

What is optimal about these studies?
MJB. The best example is a design that has long been advocated to gain absolute bioavailability and other fundamental pharmacokinetic parameters. We often refer to this as an IV-PK study. An intravenous ¹⁴C-dose is given contemporaneously with an extravascular therapeutic dose. For example volunteers may be dosed orally as usual. An IV dose is then administered at a much lower level (£ 100 mg) compared to the oral dose and is given, for example at the Tmax. The IV dose is so low that it does not contribute significantly to the overall systemic drug concentration to which the volunteer is exposed.

What is the advantage to conducting an IV-PK study in Phase 1?
MJB. An IV-PK study can characterize the effect of first pass metabolism, for example. That early insight can enable critical go-, no-go decisions for compounds which suffer from poor bioavailability and would require extensive down-stream formulation development.

You mentioned metabolite safety. How can that be investigated in Phase 1?
MJB. We've worked with our clients to design a screening approach that identifies potential human-specific or disproportionate metabolites. In this approach, we add a very low level of ¹⁴C-drug to the oral dose. We then collect plasma and excretory samples and profile them for metabolites. Using pooled samples, we can keep the analysis to a minimum thereby reducing overall cost. We refer to this study as our MIST approach, named after the US regulatory guideline.

How cost-effective are these Phase 1 approaches?
MJB. The insight provided at such an early stage has the potential to save our clients significantly by halting development of a compound with unfavorable characteristics or guide the ongoing development of an appropriate candidate.

The early insight from these studies is provided at only a small increase in the cost of a traditional Phase 1 study. In the case of the IV-PK study, the IV dose can be administered without additional non-clinical safety data. Safety data for the oral route of administration is sufficient. Furthermore, formulation of the intravenous dose at such a low level is trivial, taking intravenous formulation off the critical path. This is significant as many contemporary assets are pretty insoluble.

By using flexible protocol designs in Phase 1, these studies can be incorporated into existing Phase 1 SAD or MAD designs, or as part of a fed/fasted Phase I approach.

Are the ¹⁴C tracer/AMS combination studies in Phase 1 covered by regulations?
MJB. Yes, our metabolite screening approach is derived from FDA's 'Safety Testing of Drug Metabolites' (MIST) guidance. This approach has recently been reinforced in the new ICH M3 guidelines adopted at the beginning of this year. ICH M3 also makes provision for IV-PK studies. IV tracer studies which require only µg levels of drug do not require separate IV toxicity data.

Can the concept be applied to biologics?
MJB. Yes, Xceleron have been working with biologics for several years and has developed techniques for ¹⁴C labeling.

Michael J. Butler, Ph.D., Chief Executive Office, has 20 years experience in science-driven businesses in Europe, US and Asia. He has been President, Scientific Operations and Chief Scientific Officer with Aptuit, Group Vice President at MDS-PS and Group Director, Business Development for Huntingdon Life Sciences.