Syntheses and characterization of vitamin B₁₂–Pt(II) conjugates and their adenosylation in an enzymatic assay

Abstract

Aiming at the use of vitamin B₁₂ as a drug delivery carrier for cytotoxic agents, we have reacted vitamin B₁₂ with trans-[PtCl(NH₃)₂(H₂O)]⁺, [PtCl₃(NH₃)]⁻ and [PtCl₄]²⁻. These Pt(II) precursors coordinated directly to the Co(III)-bound cyanide, giving the conjugates [{Co}–CN–{trans-PtCl(NH₃)₂}]⁺ (5), [{Co}–CN–{trans-PtCl₂(NH₃)}] (6), [{Co}–CN–{cis-PtCl₂(NH₃)}] (7) and [{Co}–CN–{PtCl₃}]⁻ (8) in good yields. Spectroscopic analyses for all compounds and X-ray structure elucidation for 5 and 7 confirmed their authenticity and the presence of the central “Co–CN–Pt” motif. Applicability of these heterodinuclear conjugates depends primarily on serum stability. Whereas 6 and 8 transmetallated rapidly to bovine serum albumin proteins, compounds 5 and 7 were reasonably stable. Around 20% of cyanocobalamin could be detected after 48 h, while the remaining 80% was still the respective vitamin B₁₂ conjugates. Release of the platinum complexes from vitamin B₁₂ is driven by intracellular reduction of Co(III) to Co(II) to Co(I) and subsequent adenosylation by the adenosyltransferase CobA. Despite bearing a rather large metal complex on the β-axial position, the cobamides in 5 and 7 are recognized by the corrinoid adenosyltransferase enzyme that catalyzes the formation of the organometallic C–Co bond present in adenosylcobalamin after release of the Pt(II) complexes. Thus, vitamin B₁₂ can potentially be used for delivering metal-containing compounds into cells.