Methionine Can Favor DNA Platination by trans‐Coordinated Platinum Antitumor Drugs

Cisplatin (cis-DDP) and its cis-coordinated analogues, carboplatin and oxaliplatin, have been successfully used in the treatment of testicular and other solid tumors, but applications are restricted by side effects and intrinsic and acquired resistances. The discovery of trans-coordinated platinum complexes with antitumor activity provides a novel approach for cancer chemotherapy. Among several types of transplatinum complexes, trans-[PtCl2{E-HN=C(OCH3)CH3}2] (trans-EE) raised particular interest because of its higher cytotoxicity than the cis isomer and its activity towards several cis-DDP-resistant tumor cells. Mechanistic studies indicated that trans-EE has different DNA binding modes relative to cis-DDP, although their reaction rates were similar. DNAmodified by trans-EE could not be recognized by high-mobility group (HMG), the protein that interferes with DNA repair of cis-DDP adducts, whereas histone H1 could bind to trans-EE-modified DNA and prevent DNA polymerization and repair. A recent study also indicated that methionine was the preferable binding site of trans-EE in the reaction with cytochrome c, and different binding modes were observed between cisand trans-platinum complexes. Many cellular molecules, including proteins, peptides, and also some small molecules, can play significant roles in the functioning of and resistance to drugs, such as DNA platination, drug transport, and efflux. Sulfur-containing proteins are of special interest because of their high affinity for platinum, their abundance (e.g. albumin), and their involvement in metal-ion transport (e.g. the copper transporter protein CTR1, which contains methionine-rich extracellular motifs and appears to be involved in platinum-drug transport through the cell membrane). Kinetic studies indicated that the S platination of l-methionine (Met) or Nacetyl-l-methionine (AcMet) was kinetically preferred, whereas N7 coordination of guanine was thermodynamically favored. Studies using the model compound [PtCl(dien)] showed that the migration of platinum from S-Met to N7guanine (G–N7) was fairly slow (t1/2= 21–147 h at 310 K depending on DNA sequence), which was obviously slower than the direct DNA platination by [PtCl(dien)]. Although Met could slightly increase the rate of platination of cis-DDP to guanosine monophosphate (GMP), the reaction with synthetic DNA showed that the presence of Met actually inhibited platination on both single strand (ss) and double strand (ds) DNA. Herein we show that the platination rates of both GMP and DNA are substantially enhanced by a Met ligand bound to trans-EE. Moreover the reaction is highly pH-dependent. This enhancement has been observed for all nucleotides used in this work, including monomeric GMP, synthetic ssand dsDNA, and natural DNA. It has been observed that the formation of a Met intermediate is about seven times faster than G–N7 platination (Figure S1 in the Supporting Information). Accordingly, DNA platination is significantly faster via a Met intermediate (Scheme 1). On the basis of activity studies and the formation of this type of adducts also in the cellular system, it is suggested that the mechanism of trans-EE could differ substantially from that of conventional cisplatinum compounds.