Based on the above outlined considerations and existing guidelines, a new approach for toxicology assessment based upon the definition of three distinct toxicological limits is proposed: 1) <1.5 μg, 2) <100 μg and 3) >100 μg.
This limit is based on the Threshold of Toxicological Concern (TTC) concept. A TTC value of 1.5 μg/day intake of a genotoxic impurity is considered to be associated with an acceptable risk (excess cancer risk of <1 in 100,000 over a lifetime) for most pharmaceuticals. It should be noted that in “Note for guidance on impurities in new drug products” (CPMP/ICH/2738/99) a total daily intake (TDI) of an impurity may be up to 2 mg.
Based on case-by-case judgment for RPs applied in amounts of <1,5 μg injected dose, no toxicology tests may be required, however potential toxicity should be addressed and discussed. The evaluation of potential toxicity may be performed by desktop screening (in silico) and (quantitative) structure-activity relationship (Q)SAR). FDA is considering a tiered approach which ranges from <1,5 μg/person/day as a general threshold, <15 μg/person/day for chemicals without structural alerts for carcinogenicity or with negative mutagenicity test results (Ames test) and <45 μg/person/day for chemicals without structural alerts for carcinogenicity or with negative mutagenicity test results (Ames test) and a LD50 (median lethal dose) >1000 mg/kg bodyweight.
Compounds excluded from this concept: high potency genotoxic carcinogens such as aflatoxin-like-, N -nitroso-, and azoxy-compounds. Risk assessment of members of such groups requires compound-specific toxicity data [Guideline on the limits for genotoxic impurities, CPMP/SWP/5199/02, European Medicines Agency; Opinion on Use of the Threshold of Toxicological Concern (TTC) Approach for Human Safety Assessment of Chemical Substances with focus on Cosmetics and Consumer Products, SCCP/1171/08, European Commission; ICH guideline M7 on assessment and control of DNA reactive (mutagenic) impurities in pharmaceuticals to limit potential carcinogenic risk, EMA/CHMP/ICH/83812/2013, European Medicines Agency]. Any substance with positive Ames test should be evaluated on a case-by-case basis. It must be mentioned that the TTC approach allows up to 120 μg for a single dose in a recent ICH document (ICH M7) soon replacing current guidelines that suggests even room for extension of this concept [Multidisciplinary ICH guidelines including M 7].
In this case (so-called microdosing) the “Note for guidance on non-clinical safety studies for the conduct of human clinical trials and marketing authorization for pharmaceuticals” (CPMP/ICH/286/95) can be applied.
For <100 μg, approach 1 in “ICH guideline M3(R2) on non-clinical safety studies for the conduct of human clinical trials and marketing authorisation for pharmaceuticals” is applicable. Typically, a 100 x times the clinical dose in thirty animals (rodents) and examination in ten animals/sex on the day following the injection and five animals/sex after 14 days (hematology, clinical chemistry, necropsy, and histopathology) should be used. The in vivo toxicology tests must be performed in compliance with Good Laboratory Practice (GLP), interspecies scaling (allometric scaling) should be applied to calculate from animal to human dose [Note for guidance on non-clinical safety studies for the conduct of human clinical trials and marketing authorization for pharmaceuticals, CPMP/ICH/286/95, European Medicines Agency].
The 1000 x time scaling is mentioned in the same guideline approach and may be followed if the allometric scaling is not used. The exact approach should be negotiated with the competent authorities.
The 100 x allometric scaling is also in agreement with FDA’s “Guidance for industry, Developing medical imaging drugs and biological products, Part 1: conducting safety assessments”:
“We recommend that the NOAEL in safety pharmacology studies in suitable animal species be at least one hundred times (100x) greater than the maximal mass dose to be used in human studies.”
It is recommended for all approaches to use a body weight of 50 kg for a patient to be on the safe side.
A major limitation of this microdosing approach is that it does not take into account that pharmacological and toxicological effects are usually not determined by the mass but the molar amount to be applied. For larger molecules such as peptides or proteins this causes a major limitation of the microdosing concept based solely on the mass. It should be noted, that the average molecular weight (Mw) of a drug compound is approx. 300 g/mol.
Therefore, from a pharmacological point of view a peptide with a Mw of approx. 1500 g/mol having the same pharmacological potency as a low molecular weight drug could be used in a five-fold higher amount. In such a case the microdosing concept could be extended to, e.g., <500 μg and still be within the same equimolar limit. For larger molecules such as proteins the FDA’s “Guidance for Industry, Investigators, and Reviewers; Exploratory IND Studies” sets the limit to <30 nmoles: “Due to differences in molecular weights as compared to synthetic drugs, the maximum dose for protein products is ≤30 nanomoles.” This corresponds to <100 μg of a drug having a molecular weight of 300 g/mol.
It also should be noted that for radiopharmaceuticals, usually extensive biodistribution data (often including imaging) from preclinical studies are available. These studies give detailed quantitative data on accumulation and elimination in tissues and excretion pathways. Based on such data the design of extended single dose toxicity studies may be focused on risk organs and tissues, thereby eliminating the requirement especially for (high cost) histopathological data in all organs, focusing on main organs where the RP accumulates by, e.g., binding to receptors or other target structures. Such design has to be made on a case-by case basis and its rational being described in detail in the application process.
Applications of more than 100 μg substance in the case of RPs may be required for imaging with peptides and proteins or for therapeutic applications. The assumptions made focus on single dose applications in clinical studies, i.e., the RP is expected to be cleared completely from the blood before the next application.
In this case, also the “Note for guidance on non-clinical safety studies for the conduct of human clinical trials and marketing authorization for pharmaceuticals” can be applied using approach 3. This includes also an extended single dose toxicity study, however, in this case in rodent and non-rodent species, as well as a test for genetoxicity (usually Ames test). Selection of histopathology data may be based on the same considerations as described in the <100 μg group. The toxicology tests must be performed in compliance with Good Laboratory Practice (GLP).
For the anticancer use of RPs (immunoradiotherapy, endoradiotherapy) the “ICH guideline S9 on nonclinical evaluation for anticancer pharmaceuticals” can be employed. In these cases in particular genetoxicity testing for Phase I and II studies may not be required.
In accordance with scenario in section <100 μg, the argument of molar equivalence may be used.