Localized in vivo prodrug activation using radionuclides

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 exposure to activated chemotherapy in off-target sites. MethodsWe screened the ability of radionuclides to chemically activate a model radiation-activated prodrug consisting of the microtubule destabilizing monomethyl auristatin E caged by a radiation-responsive phenyl azide ("caged-MMAE") and interpreted experimental results using the radiobiology computational simulation suite TOPAS-nBio. RAiDER was evaluated in syngeneic mouse models of cancer using fibroblast activation protein inhibitor (FAPI) agents 99mTc-FAPI-34 and 177Lu-FAPI-04, the prostate-specific membrane antigen (PSMA) agent 177Lu-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. ResultsRAiDER efficiency varied by 250-fold across radionuclides (99mTc>177Lu>64Cu>68Ga>223Ra>18F), yielding up to 1.22{micro}M prodrug activation per Gy of exposure from 99mTc. Computational simulations implicated low-energy electron-mediated free radical formation as driving prodrug activation. Clinically relevant radionuclide concentrations chemically activated caged-MMAE restored its ability to destabilize microtubules and increased its cytotoxicity by up to 600-fold compared to non-irradiated prodrug. Mice treated with 99mTc-FAPI-34 and caged-MMAE accumulated up to 3000x greater concentrations of activated MMAE in tumors compared to other tissues. RAiDER with 99mTc-FAPI-34 or 177Lu-FAPI-04 delayed tumor growth, while monotherapies did not (P<0.03). Clinically-guided dosimetry suggests sufficient radiation doses can be delivered to activate therapeutically meaningful levels of prodrug. ConclusionThis proof-of-concept study shows that RAiDER is compatible with multiple radionuclides commonly used in nuclear medicine and has the potential to 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 biological and therapeutic responses. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=136 SRC="FIGDIR/small/606075v1_ufig1.gif" ALT="Figure 1"> View larger version (33K): org.highwire.dtl.DTLVardef@18aef37org.highwire.dtl.DTLVardef@5f36eforg.highwire.dtl.DTLVardef@10fa0ccorg.highwire.dtl.DTLVardef@105c416_HPS_FORMAT_FIGEXP M_FIG C_FIG