Vitamin B₁₂ as a carrier for targeted platinum delivery: in vitro cytotoxicity and mechanistic studies

Abstract

It is attractive to use vitamin B₁₂ as a carrier for targeted delivery of cytotoxic agents such as platinum complexes owing to the high demand for vitamin B₁₂ by fast proliferating cells. The basic {B₁₂–CN–Pt^II} conjugates are recognized by intracellular enzymes and converted to coenzyme B₁₂ in an enzymatic adenosylation assay. The reductive adenosylation of {B₁₂–CN–Pt^II} conjugates leads to the release of the Pt^II complexes; thus, {B₁₂–CN–Pt^II} conjugates can be considered as prodrugs. It is important not only to elucidate the activity of the cisplatin–B₁₂ conjugates, but also to understand the mode of action on a molecular level. Chemical reduction of {B₁₂–CN–Pt^II} conjugates with cobaltocene yielded cob(II)alamin and induced release of the corresponding Pt^II species. Kurnakov tests and coordination of 2′-deoxyguanosine or GMP to the released Pt^II complexes allowed isolation and characterization of Pt^II complexes as released during enzymatic adenosylation. The biological activity of these Pt^II complexes was evaluated. Since the cleaved Pt^II complexes show cytotoxicity, the {B₁₂–CN–Pt^II} conjugates can be used for specific targeting of cancer cells and therapeutic drug delivery. Preliminary in vitro cytotoxicity studies indicated lower activity (IC₅₀ between 8 and 88 μM) than found for pure cisplatin. Since active transport and receptor-mediated uptake limits the intracellular {B₁₂–CN–Pt^II} concentration, comparison with pure cisplatin is of limited use. We could show that the Pt^II complexes cleaved from B₁₂ exerted a cytotoxicity comparable to that of cisplatin itself. Cytotoxicity studies in vitamin B₁₂ free media showed a dependence on the addition of transcobalamin II for B₁₂–Pt(II) conjugates.