Localized In Vivo Prodrug Activation Using Radionuclides

Abstract

Radionuclides used for imaging and therapy can show high molecular specificity in the body with appropriate targeting ligands. We hypothesized that local energy delivered by molecularly targeted radionuclides could chemically activate prodrugs at disease sites while avoiding activation in off-target sites of toxicity. As proof of principle, we tested whether this strategy of radionuclide-induced drug engagement for release (RAiDER) could locally deliver combined radiation and chemotherapy to maximize tumor cytotoxicity while minimizing off-target exposure to activated chemotherapy. Methods: We screened the ability of radionuclides to chemically activate a model radiation-activated prodrug consisting of the microtubule-destabilizing monomethyl auristatin E (MMAE) caged by a radiation-responsive phenyl azide, and we interpreted experimental results using the radiobiology computational simulation suite TOPAS-nBio. RAiDER was evaluated in syngeneic mouse models of cancer using the fibroblast activation protein inhibitor (FAPI) agents [^99mTc]Tc-FAPI-34 and [¹⁷⁷Lu]Lu-FAPI-04 and the prostate-specific membrane antigen (PSMA) agent [¹⁷⁷Lu]Lu-PSMA-617, combined with caged MMAE or caged exatecan. Biodistribution in mice, combined with clinical dosimetry, estimated the relationship between radiopharmaceutical uptake in patients and anticipated concentrations of activated prodrug using RAiDER. Results: RAiDER efficiency varied by 70-fold across radionuclides (^99mTc > ¹¹¹In > ¹⁷⁷Lu > ⁶⁴Cu > ³²P > ⁶⁸Ga > ²²³Ra > ¹⁸F), yielding up to 320 nM prodrug activation/Gy of exposure from ^99mTc. Computational simulations implicated low-energy electron–mediated free radical formation as driving prodrug activation. Radionuclide-activated caged MMAE restored the prodrug’s ability to destabilize microtubules and increased its cytotoxicity by up to 2,600-fold that of the nonactivated prodrug. Mice treated with [^99mTc]Tc-FAPI-34 and caged MMAE accumulated concentrations of activated MMAE that were up to 3,000 times greater in tumors than in other tissues. RAiDER with [^99mTc]Tc-FAPI-34 or [¹⁷⁷Lu]Lu-FAPI-04 delayed tumor growth, whereas monotherapies did not (P < 0.003). Clinically guided dosimetry suggests sufficient radiation doses can be delivered to activate therapeutically meaningful levels of prodrug. Conclusion: This proof-of-concept study shows that RAiDER is compatible with multiple radionuclides commonly used in nuclear medicine and can potentially improve the efficacy of radiopharmaceutical therapies to treat cancer safely. RAiDER thus shows promise as an effective strategy to treat disseminated malignancies and broadens the capability of radiopharmaceuticals to trigger diverse biologic and therapeutic responses.