This contribution features a small molecule that binds TrkC (tropomyosin receptor kinase C) receptor that tends to be overexpressed in metastatic breast cancer cells but not in other breast cancer cells. A sensitizer for (1)O2 production conjugated to this structure gives 1-PDT for photodynamic therapy. Isomeric 2-PDT does not bind TrkC and was used as a control throughout; similarly, TrkC- cancer cells were used to calibrate enhanced killing of TrkC+ cells. Ex vivo, 1- and 2-PDT where only cytotoxic when illuminated, and 1-PDT, gave higher cell death for TrkC+ breast cancer cells. A 1 h administration-to-illumination delay gave optimal TrkC+/TrkC--photocytotoxicity, and distribution studies showed the same delay was appropriate in vivo. In Balb/c mice, a maximum tolerated dose of 20 mg/kg was determined for 1-PDT. 1- and 2-PDT (single, 2 or 10 mg/kg doses and one illumination, throughout) had similar effects on implanted TrkC- tumors, and like those of 2-PDT on TrkC+ tumors. In contrast, 1-PDT caused dramatic TrkC+ tumor volume reduction (96% from initial) relative to the TrkC- tumors or 2-PDT in TrkC+ models. Moreover, 71% of the mice treated with 10 mg/kg 1-PDT (n = 7) showed full tumor remission and survived until 90 days with no metastasis to key organs.
Keywords: histochemistry; imaging; metastatic breast cancer; photodynamic therapy (PDT); theranostic; tropomyosin receptor kinase C (TrkC).