Tal Burt, Robert D. Combes, in The History of Alternative Test Methods in Toxicology, 2019
Microdosing, or phase-0 clinical trials, is a regulatory pathway to first-in-human (FIH) studies that employs sub-therapeutic pharmacological exposures, to study the effects of novel drugs in humans. Microdosing is based on two main principles: first, that human data are superior to animal and in-vitro data when modelling the effects of human disease and therapeutics, and, second, that data obtained with limited pharmacological exposure can be reliably extrapolated to the therapeutic dose range.
These approaches have several operational advantages, most notably, the implied safety, which in turn is translated into quicker and less-expensive human testing, and reduced testing in animals. While these approaches were originally used primarily to study pharmacokinetics (PK), the direction and interest in phase-0/microdosing has recently been broadened to include pharmacodynamics (PD), focusing on biomarkers of efficacy and toxicity, and information relevant to drug mode of action (MoA; 18).
The term ‘in humano’ has recently been coined to describe such studies as pre-clinical trials in humans, in the vein of terms such as ‘in vivo’, ‘in vitro’ and ‘in silico’ (18). Notwithstanding the lack of therapeutic intent, these approaches produce sufficient pharmacological exposure to quantify the PK and PD properties of test items, including concentrations in target organs and tissues, binding to receptors and post-receptor modulation (19). Such approaches can include non-invasive imaging, with the test compound labelled with radionuclides, for monitoring and studying drug effects in real-time, with the potential to replace some experiments on animals, including non-human primates (NHPs; 10, 20). Phase-0/microdosing studies have also been called ‘exploratory clinical trials’ and ‘exploratory Investigational New Drug’ (eIND) trials (21, 22).
4.2 Volunteer and Clinical Studies
The ethical use of humans, particularly for the efficacy and safety testing of pharmaceuticals (23), together with biomonitoring, has long been an important way of obtaining information of direct relevance to the development of therapeutics, and there have been many discussions as to how this could be expanded without compromising volunteer safety, especially in the wake of FIH studies with serious adverse outcomes (3, 24, 25; see later, under Safer development). Phase-0/microdosing offers the possibility of testing novel drugs in human volunteers in a safer manner than in traditional phase-1 studies, and hence with less prior animal testing (26). The exceedingly small amounts of a substance can still be detected by using ultra-sensitive analytical methods (5, 19, 27; and see later, under Analytical methodologies, 4, 19, 27–29; see also, 23).
4.3 Definitions of Phase-0/Microdosing Approaches
Phase-0/microdosing approaches are defined in the International Conference on Harmonisation, Guidance on Nonclinical Safety Studies for the Conduct of Human Clinical Trials and Marketing Authorisation for Pharmaceuticals M3 (ICH M3 guidelines; 22). The document contains five approaches under the category of ‘exploratory clinical trials’ (synonymous with phase-0/microdosing trials), but also emphasises that other, intermediate approaches on the continuums of dosage and duration of exposure are possible (Fig. 1).
The first approach involves a total dose (single or divided) of <100 μg, supported by a 14-day extended single-dose toxicity study in one species, usually a rodent, by the clinical route of administration. A second microdosing approach permits a total of <500 μg per subject, divided into five doses, each no more than 100 μg and separated by at least six half-lives. This approach should be supported by a 7-day repeated-dose toxicity study in one species, usually a rodent. Neither approach requires genotoxicity studies. The other approaches are described in Table 2.
Table 2. Phase-0, Including Microdosing Approaches From ICH M3 Guidelines (ICH 2009). Intermediate Forms are Possible and the Optimal Approach Should be Arrived at Based on Discussions With Local Regulators
|Approach 1||Approach 2||Approach 3||Approach 4||Approach 5|
|Dose definition||≤1/100th NOAEL and ≤1/100th of pharmacologically active dose (scaled on mg/kg for IV and mg/m2 for oral)||Same as approach 1||Starting at subtherapeutic dose and moving into the anticipated therapeutic range but < ½ NOAEL||Starting dose: <1/50 of NOAEL AUC; into the anticipated therapeutic range, but <10th pre-clinical AUC if no toxicity, or < NOAEL||Starting dose: <1/50 NOAEL; into the anticipated therapeutic range, but < non-rodent NOAEL AUC, or <½ rodent NOAEL AUC|
|Cumulative dose||100 μg||500 μg|
|Limit per dose||100 μg||100 μg|
|Maximal daily dose||100 μg||100 μg|
|Number/duration of dosing||1|
Could be divided into multiple doses with a total of 100 μg
|Washout||No washout||Six or more half-lives between doses||No washout||No washout||No washout|
|Pharmacology||In vitro and receptor profiling|
PD model supporting human dose
|Same as Approach 1||Same as Approach 1 + Core battery of safety pharmacology||Same as Approach 1 + Core battery of safety pharmacology||Same as Approach 1 + Core battery of safety pharmacology|
|General toxicity studies||14-day extended single dose toxicity||7-day repeated-dose toxicity||Extended single-dose toxicity; in rodent and non-rodent||14-day repeated-dose toxicity in rodent and non-rodent||14-day repeated-dose toxicity in rodent and non-rodent|
|Genotoxicity studies||Not recommended|
SAR included, if available
|Same as Approach 1||Ames assay||Ames assay + chromosomal damage test||Ames assay + chromosomal damage test|
|Dosimetry estimates||For highly radioactive agents||Same as Approach 1||Same as Approach 1||Same as Approach 1||Same as Approach 1|
AUC, area under the curve; GLP, good laboratory practice; NOAEL, no observed adverse effect level.
Adapted from Burt, T., John, C.S., Ruckle, J.L., et al. (2016). Phase-0/microdosing studies using PET, AMS, and LC-MS/MS: A range of study methodologies and conduct considerations. Accelerating development of novel pharmaceuticals through safe testing in humans – a practical guide. Expert Opinion on Drug Delivery, 4, 657–672.
4.4 Reception and Regulatory Position
4.4.1 A Mixed Reception
While phase-0/microdosing approaches have been welcomed by many, for both scientific and animal welfare reasons, they are not without some controversy, and in particular, with regard to concerns over the extrapolation of study data from sub-therapeutic to therapeutic levels of exposure. In other words, doubt has been expressed as to whether a substance in tiny amounts behaves in the same way, pharmacologically, as it does in larger amounts (30, 31; and see later, under Extrapolating phase-0/microdose study results).
4.4.2 Regulatory Interest
The phase-0/microdosing framework was endorsed by regulatory authorities, including the European Agency for the Evaluation of Medicinal Products (4), the US Food and Drug Administration (FDA; 21) and Japanese (Ministry of Health, Labour and Welfare; 32) and was subsequently internationally harmonised and adopted by other authorities as well (22). This is compatible with the recommendations in the FDA’s oft-quoted and much-welcomed 2004 report on innovation and stagnation in clinical development, which called for a greater emphasis on in vitro, computational and clinical tools, one of which was said to be microdosing (33, 34).
The FDA has been explicit in its encouragement of flexibility and efficiency in FIH studies and IND applications, stating the following in the introduction to the 2006 Exploratory eIND Guidelines, the first to define phase-0/microdosing approaches in the USA: Existing regulations allow a great deal of flexibility in the amount of data that needs to be submitted with an IND application, depending on the goals of the proposed investigation, the specific human testing proposed, and the expected risks. The Agency believes that sponsors have not taken full advantage of that flexibility and often provide more supporting information in INDs than is required by regulations (21).