Radiopharmaceutical Validation for Clinical Use

Abstract

Radiopharmaceuticals are reemerging as attractive anticancer agents, but there are no universally adopted guidelines or standardized procedures for evaluating agent validity before early-phase trial implementation. To validate a radiopharmaceutical, it is desirous for the radiopharmaceutical to be specific, selective, and deliverable against tumors of a given, molecularly defined cancer for which it is intended to treat. In this article, we discuss four levels of evidence-target antigen immunohistochemistry, in vitro and in vivo preclinical experiments, animal biodistribution and dosimetry studies, and first-in-human microdose biodistribution studies-that might be used to justify oncology therapeutic radiopharmaceuticals in a drug-development sequence involving early-phase trials. We discuss common practices for validating radiopharmaceuticals for clinical use, everyday pitfalls, and commonplace operationalizing steps for radiopharmaceutical early-phase trials. We anticipate in the near-term that radiopharmaceutical trials will become a larger proportion of the National Cancer Institute Cancer Therapy Evaluation Program (CTEP) portfolio.

Keywords: nuclear medicine; preclinical; radiation oncology; radiopharmaceutical; validation.

Figure 1

Hierarchy of evidence to justify radiopharmaceutical target-driven early-phase combination trial designs. (A) Step 1 of radiopharmaceutical target validation uses tissue microarray (TMA) immunohistochemistry to distinguish tumor antigens of interest from amongst positive and negative controls. (B) Step 2 involves two in vitro cell survival experiments followed by two in vivo tumor growth delay experiments in animal models [e.g., zebrafish (Danio rerio) or mouse (Mus musculus)]. (C) Step 3 is a single rodent or nonrodent species biodistribution study for pharmacokinetics and dosimetry. (D) Step 4 is a first-in-human radiation/nuclear medicine imaging experience that precedes early-phase or late phase clinical development. Sample size (N) represents estimated cohort sizes.

Figure 2

Radiopharmaceutical-drug screen for in vivo cytotoxic effect size using zebrafish (Danio rerio). (A) Listed are at least eight of the possible treatment schemes for a three-agent oncology therapeutic drug screen. To test each treatment in higher order vertebrates (like 40 mice) would be expensive, and, an up to 3-month endeavor. Oppositely, to test each treatment in zebrafish (like 12 fish per treatment from a single 100-embryo clutch) is inexpensive, robust, and finishes in 14 days or less. (B) Depicted are basic developmental milestones for zebrafish postfertilization, from embryo to larvae (72 h) to juvenile fish (14 days). Unperturbed zebrafish development is linear. Treatment (arrows) stalls maturation versus time, with greater treatment effect proportional to protracted growth and maturation. Teratogenic treatment effects are also readily apparent. (C) Zebrafish-screened “effective” treatments are carried forward to conventional, reduced total animal number, rodent or nonrodent tumor growth delay assays.

Figure 3

Radiopharmaceutical target-driven early-phase enrichment trial designs. (A) Early-phase 0 enrichment trial designs evaluate a new treatment only in a theranostic target-positive subject subpopulation. (B) Early-phase I trial designs for agent safety as a primary objective in an ‘all-comer’ approach might otherwise assign both target-positive and target-negative patients to a radiopharmaceutical treatment under investigation. Diagnostic imaging means baseline and posttherapy conventional radiation/nuclear medicine imaging for initial exploratory objective response assessment in either trial design.

Figure 4

Radiopharmaceutical target-driven phase II combination trial designs isolating treatment effects. (A) Enrichment phase II trial designs evaluate a diagnostic agent in a theranostic pair as a triage step in all patients. Random allocation applies only to patients with “theranostic-positive” results. (B) Theranostic pair-stratified designs randomly allocate both “theranostic-positive” and “theranostic-negative” patients to the radiopharmaceutical-based treatment under investigation. Diagnostic imaging means baseline and posttherapy conventional radiation/nuclear medicine imaging for objective response assessments.