Linker stability influences the anti-tumor activity of acetazolamide-drug conjugates for the therapy of renal cell carcinoma

Small molecule-drug conjugates (SMDCs) are increasingly being considered as an alternative to antibody-drug conjugates (ADCs) for the selective delivery of anticancer agents to the tumor site, sparing normal tissues. Carbonic anhydrase IX (CAIX) is a membrane-bound enzyme, which is over-expressed in the majority of renal cell carcinomas and which can be efficiently targeted in vivo, using charged derivatives of acetazolamide, a small heteroaromatic sulfonamide. Here, we show that SMDC products, obtained by the coupling of acetazolamide with monomethyl auristatin E (MMAE) using dipeptide linkers, display a potent anti-tumoral activity in mice bearing xenografted SKRC-52 renal cell carcinomas. A comparative evaluation of four dipeptides revealed that SMDCs featuring valine-citrulline and valine-alanine linkers exhibited greater serum stability and superior therapeutic activity, compared to the counterparts with valine-lysine or valine-arginine linkers. The most active products substantially inhibited tumor growth over a prolonged period of time, in a tumor model for which sunitinib and sorafenib do not display therapeutic activity. However, complete tumor eradication was not possible even after ten intravenous injection. Macroscopic near-infrared imaging procedures confirmed that ligands had not lost the ability to selectively localize at the tumor site at the end of therapy and that the neoplastic masses continued to express CAIX. The findings are of mechanistic and of therapeutic significance, since CAIX is a non-internalizing membrane-associated antigen, which can be considered for targeted drug delivery applications in kidney cancer patients.