Structural Elucidation of Cisplatin and Hydrated cis-Diammineplatinum(II) Complex Conjugated with Cyanocobalamin by Liquid Chromatography with Electrospray Ionization-Mass Spectrometry and Multistage Mass Spectrometry

Ligand-Tuning of the Stability of Pd(II) Conjugates with Cyanocobalamin

Besides the well-known functions performed by vitamin B₁₂ (CblCN) in biochemical processes of the human body, an increasing interest has been raised by the possibility of its use as a transmembrane drug carrier, capable, among others, of enhancing the accumulation of inorganic cytostatics in cancer cells. The present study was aimed at determining the possibility of the formation of CblCN conjugates with Pd(II) complexes. A key aspect was their stability, which we attempted to tune by appropriate choice of ligands. Syntheses, spectroscopic analysis of postreaction systems and kinetic investigations of conjugate formation reactions, have been complemented by DFT modelling. The obtained results showed that ligand charge, geometry and electron affinity may have a significant impact on carrier binding and release leading to the activation of the Pd(II) complex. This provides a rationale to expect that with appropriate composition of the coordination sphere, it will be possible to extend the spectrum of less toxic inorganic chemotherapeutics.

In vitro reactions of a cyanocobalamin-cisplatin conjugate with nucleoside monophosphates

RATIONALE

Cisplatin (CP) is a widely used anticancer drug characterized by toxic side effects that could be alleviated using novel delivery systems including CP prodrugs. The in vitro incubation of a putative prodrug, obtained from cyanocobalamin (CNCbl) and cis-diamminemonochloroplatinum(II) (mCP), with nucleoside monophosphates (NMPs) was investigated.

METHODS

The in vitro reactions between the putative prodrug CNCbl-mCP and the NMPs of adenosine (AMP), guanosine (GMP), cytidine (CMP) and uridine (UMP) were carried out in slightly acidic water-methanol solutions at 37°C for 24 h. Each sample was examined using reversed-phase liquid chromatography coupled with electrospray ionization in positive ion mode and tandem mass spectrometry (RPLC/ESI-MS/MS) by collision-induced dissociation in a linear ion-trap mass spectrometer.

RESULTS

Seven adducts were recognized as formed by substitution reactions of the chloride ligand in planar CP. Comparison between observed and theoretical isotopic patterns together with MS/MS fragmentation pathways revealed the presence of single or multiple binding sites depending on the NMP involved. The CNCbl-mCP conjugate was found to interact with N⁷ or O⁴ atoms of GMP and UMP, respectively, generating single adducts, while two isomeric adducts were observed for CMP. Finally, AMP gave rise to three isomeric adducts.

CONCLUSIONS

In agreement with literature data relevant to the interaction between CP and NMPs, the most reactive nucleotides were AMP and GMP. The present RPLC/ESI-MS/MS approach is very promising for investigation of the reactions of CP conjugates with ribonucleotides not only in vitro but also in vivo.

Tandem mass spectrometry characterization of a conjugate between oleuropein and hydrated cis-diammineplatinum(II)

RATIONALE

Oleuropein (Ole) has been claimed to mitigate cisplatin (CP)-induced acute injury in kidney and liver of mice. In vitro reactivity of hydrated CP species with Ole, and an Ole metabolite, hydroxytyrosol (HT), is of great interest as the preliminary step for gathering in vivo information on the possible physiological role of the Ole/HT-cis-diammineplatinum(II) (Ole/HT-cis-DAP) conjugate.

METHODS

Reversed-phase liquid chromatography coupled to electrospray ionization mass spectrometry using a linear ion trap instrument (RPLC/ESI-MS) and tandem mass (MS/MS) measurements, both in positive and negative ion mode, revealed the molecular identity of platinum-based conjugates.

RESULTS

The Ole-cis-DAP conjugate (i.e., C₂₅ H₃₆ N₂ O₁₃ Pt^II ) features two cis-ammine non-leaving ligands and a bidentate catechol ligand moiety belonging to Ole; the coordination of the central Pt(II) is square-planar with non-equivalent bond angles compared with the ideal arrangement of 90°. HT, the free Ole metabolite excreted in human urine, acts as bidentate O,O-donor ligand of cis-DAP as well.

CONCLUSIONS

The first evidence, together with structural information, is provided about the in vitro formation of a conjugate between cis-DAP and Ole or its urinary metabolite HT. Presuming that such conjugates are also generated in vivo, the mechanisms by which they might contribute to reduce CP toxicity remain to be elucidated.