Exploration of a Pretargeted Theranostic Copolymer Employing Inverse Electron-Demand Diels-Alder Conjugation in Ovarian Cancer

Similar to radioimmunotherapeutics, nanomedicine-based radiotherapeutics are hampered by dose-limiting radiotoxicities due to long blood circulation times. To address this limitation, we explored the pretargeting approach using the inverse electron-demand Diels-Alder (IEDDA) reaction between trans-cyclooctene (TCO) and tetrazine (TZ). Specifically, we synthesized low (22.7 kDa)- and high (93.7-108.7 kDa)-molecular-weight N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers with TCO-incorporated linkers of various lengths in parallel with small-molecule Lu-177-labeled TZ chaser agents. The IEDDA reaction kinetics between the TCO-incorporated copolymer and TZ chaser agents was found to be initially rapid but quickly decreased as copolymer molecular weight increased. The various linkers employed in this study were found to significantly impact IEEDA kinetics but did not yield a clear trend between length and kinetics. The pretargeted system was examined in an OVCAR-3 ovarian cancer mouse model. The tumor uptake of the TCO-incorporated copolymers by 76 h ranged from 10 to 14.1%ID/g, but the elevated residual blood retention of the high-molecular-weight copolymers led to undesirable IEDDA reaction and retention in the blood compartment. The high off-target blood retention resulted in modest tumor uptake and poor tumor-to-nontarget tissue ratios at 4 h postadministration of the chaser agent. A masking agent (DP-TZ) that reacts with and inactivates TCO moieties in the blood compartment was found to substantially lower the undesirable retention of the Lu-177-labeled TZ chaser agent in the blood, leading to significant improvements in tumor-to-nontarget ratios.