FIRST-IN-CLASS X-RAY-ACTIVATABLE PARPi PRODRUGS FOR SPATIOTEMPORAL CONTROL OF PARP1 INHIBITION IN SOLID TUMORS: DESIGN AND IN SILICO EVALUATION

Poly(ADP-ribose) polymerase inhibitors (PARPi) are highly effective agents that induce synthetic lethality in homologous recombination-deficient tumors, including triple-negative breast, ovarian, and endometrial cancers. However, their limited selectivity results in significant systemic toxicities, restricting their clinical applicability [1]. In the context of anticancer therapy, a growing frontier in medicinal chemistry is represented by the development of advanced prodrug systems designed as externally activatable therapeutic tools. In this regard, different external stimuli are currently being investigated, including light, ionizing radiation, and ultrasound [2]. Among these, X-ray-responsive systems are particularly attractive due to their deep tissue penetration and their established clinical use in radiotherapy (RT), enabling localized drug activation within irradiated tumor tissues and synergistic therapeutic effects [2–4]. PARPi are currently used in combination with RT in established therapeutic protocols, where ionizing radiation enhances DNA damage and potentiates PARP inhibition. This therapeutic interplay provides the rationale for the development of X-ray-activatable PARPi prodrugs. Herein, we report the rational design of the first X-ray-activatable prodrugs of clinically approved PARP inhibitors (Figure 1). In details, a comprehensive analysis of available PARP1-ligand crystallographic structures was carried out to identify key and recurrent binding interactions and to define suitable positions for chemical modification. On this basis, a focused library of putative prodrugs was generated by selectively masking critical interaction motifs to enforce an inactive OFF-state. Different classes of radiation-responsive caging groups were introduced, including azides, N-oxides, N-alkylpicolinium, dihydroxybenzyl, azo linkers, and metal-based cages, all known to undergo cleavage upon exposure to ionizing radiation. The impact of these modifications on PARP1 binding was systematically evaluated through in silico studies, assessing the extent to which target engagement is suppressed prior to activation, together with the prediction of key ADMET properties.