Cobalt hematoporphyrin inhibits CLEC-2-podoplanin interaction, tumor metastasis, and arterial/venous thrombosis in mice

The platelet activation receptor C-type lectin-like receptor 2 (CLEC-2) interacts with podoplanin on the surface of certain types of tumor cells, and this interaction facilitates tumor metastasis. CLEC-2 is also involved in thrombus formation and its stabilization. Because CLEC-2-depleted mice are protected from experimental lung metastasis and thrombus formation and do not show increased bleeding time, CLEC-2 may serve as a good target for antimetastatic or antithrombotic drugs. We screened 6770 compounds for their capability to inhibit CLEC-2-podoplanin binding using an enzyme-linked immunosorbent assay. In the first screening round, 63 compounds were identified and further evaluated by flow cytometry using CLEC-2-expressing cells. We identified protoporphyrin IX (H2-PP) as the most potent inhibitor and modified its hematoporphyrin moiety to be complexed with cobalt (cobalt hematoporphyrin [Co-HP]), which resulted in an inhibitory potency much stronger than that of H2-PP. Surface plasmon resonance analysis and molecular docking study showed that Co-HP binds directly to CLEC-2 at N120, N210, and K211, previously unknown podoplanin-binding sites; this binding was confirmed by analysis of CLEC-2 mutants with alterations in N120 and/or K211. Co-HP at a concentration of 1.53 mM inhibited platelet aggregation mediated through CLEC-2, but not that mediated through other receptors. IV administration of Co-HP to mice significantly inhibited hematogenous metastasis of podoplanin-expressing B16F10 cells to the lung as well as in vivo arterial and venous thrombosis, without a significant increase in tail-bleeding time. Thus, Co-HP may be a promising molecule for antimetastatic and antiplatelet treatment that does not cause bleeding tendency.