Hydrolysis of Cisplatin

Table 22 The energy of the transition state compared to the initial energy for the hydrolysis of cisplatin in various solvents. All results are from ref. 77... 

Fig. 3 Evaluated steps in the hydrolysis of cisplatin by CPMD calculations...

The free chloride concentration inside cells is much lower (some 4 mM) and under these conditions hydrolysis of cisplatin can take place, albeit slowly. In fact the most important hydrolysis product is the [PtCl(H20)(NH3)2] cation which has a pK value of 6.5, and so above pH = 6 it starts to ionize to form [PtCl(OH)(NH3)2] The cationic Pt species with a labile aqua ligand is much more reactive than cisplatin, so the monoaqua species is most likely to react with DNA and other molecules in the cell. Carboplatin hydrolyses much slower, and far less is known in this case. Studies of this type for oxaliplatin, are also still lacking. Transport of platinum compounds through the nuclear membrane is also hardly studied. Whether a special nuclear localizing signal peptides (NLS) may play a role remains uncertain... 

Fig. 2. Ligand substitution as a prodrug strategy for metallochem-otherapeutics (a) general scheme of prodrug activation by ligand substitution hydrolysis of a metal—halide bond is a typical activation pathway of metal-based anticancer drugs, as exemplified by the activation of cisplatin (b) and a ruthenium—arene complex (c).

The bifunctional amine-tethered ruthenium(II) arene complexes [Ru(r6 ti1-C6H5CH2(CH2)i1NH2)C12] (n = 1,2) (13a,b) show two consecutive hydrolysis steps to yield the mono- and bis-aqua complexes (64). At extracellular chloride concentrations, the majority of the complexes could be expected to be present as the mono-aqua adduct. Equilibrium constants were determined for both steps (for 13b, Ki = 145 mM K2 = 5.4 mM) and found to be considerably higher than those of cisplatin, which also has two reactive sites available. 

The conversion of the monofunctional adducts into bifunctional lesions depends drastically on the structure of the Pt drug. Obviously, Pt compounds exhibiting trans geometry form different bisadducts than cisplatin and hence, a different spectrum of antitumor activity is expected. Mechanistically, the formation and possible isomerization of bisadducts are not well understood. The assumption that hydrolysis of the second leaving group controls the formation of bisadduct may be an oversimplification. Studies with model compounds as well as with oligonucleotides have indicated that a certain nucleobase may be a powerful nucleophile toward Pt(II) if spatially in a correct position. Unfortunately, our knowledge on these interactions is at present very limited. 

Rate constants determined during the reaction of cisplatin with DNA. b kla corresponds to the hydrolysis of CL trans to the c-CeHnNH2, and kib to the hydrolysis of Cl" trans to NH3. 

This route gives a much better yield and a purer compound than when K2[PtCl4] is treated with ammonia directly. A disadvantage, however, is the necessity to use silver salts (usually nitrate) with overnight stirring, resulting in the possibility of side products formed by hydrolysis of the intermediate aqua species cis-[Pt(NH3)2(H20)2]2+.12 We here present a rapid and facile one-step synthesis of cisplatin. The experimental conditions are based on Lebedinsky s method,8 slightly modified as specified. 

Fig. 6. Hydrolysis scheme of cisplatin the final dimerization (or even trimerization) occurs only at high pH and in concentrated solutions8,35)...

Studies of cisplatin bound to DNA have also been reported by others. Eastman111,112) has used radiolabeled PtCl2(en) to detect and quantify the adducts after enzymatic hydrolysis. Johnson113,114) has used an acid-catalyzed depurination followed by electrophoresis. Rahn115) used radiolabeled platinum compounds for detection, whereas Olinsky and Walter116) have used the SnCl2 method to quantify the amounts of the various cisplatin-DNA adducts. 

P. Shearan, J. M. F. Alvarez, N. Zayed, and M. R. Smyth, HPI separation of cisplatin and its hydrolysis products on aluminudi and application to studies of their interaction with cysteine, Bio [ med. Chromatogr., 4 78 (1990). 

Platinum speciation using IPC-ICP-MS has been achieved by Zhao et al. [50], An ODS C18 column and 1-heptanesulfonate ion-pairing reagent at pH 2.6 were used to separate the metabolites of cisplatin and cisplatin hydrolysis products. The low pH was required in order to retain thiol containing complexes. All complexes were resolved and urine and blood samples were analyzed by the speciation method. 

Gold(III) organometallics are isoelectronic with cisplatin-like Ptn complexes. For example, complex 10 hydrolyses in water [123] and has shown activity in human tumour xenograft models [124]. However, its mechanism of action is different from that of cisplatin [124,125]. The role of hydrolysis as an activation step for this class of compounds is not yet clear. 

The mechanism of action of cisplatin is believed to involve activation via hydrolysis inside cells where the Cl- concentration is much lower (ca. 4 him) than outside cells (ca. 100 him) [10]. Ptn-OH2 bonds are more reac-... 

The time-dependence of the [1H,15N] 2D-NMR spectra can also be used to determine the hydrolysis rates for each individual chloride ligand in the dichloride complex and in the monoaqua monochloro complex, by fitting the curves of the concentration changes of each species with time [14], The NMR spectrum of 15N-cisplatin in water at 310 K for 40 h (at equilibrium) contains [1H,15N] resonances assignable to unreacted cisplatin, the monoaqua and diaqua adducts in a ratio of 0.64 0.35 0.01, respectively, from which an equilibrium constant of 2.72 for the first stage of cisplatin hydrolysis was calculated [13]. 

The reaction of cA-[Pt(NH3)2(H20)2]2+ (6), a hydrolysis product of cisplatin, with glycine under mildly acidic conditions, likewise leads to complexes 7 and 8 in which glycine is bound monodentate through oxygen only (Scheme 2). An isomerization occurs, slowly in acidic solution, faster at higher pH, to give the thermodynamically more stable chelate complex 9 [3-5]. 

Since the discovery of the antitumor activity of m-PtCNFhuC (cisplatin, cd-DDP) by Rosenberg et al. [1], the interactions of cisplatin with nucleotides and nucleobases have attracted attention towards gaining an understanding of the mechanism of the antitumor activity of cisplatin at a molecular level. In the course of such studies, dark-blue platinum complexes called platinum blues were obtained when hydrolysis products of cis-... 

Ab initio molecular-dynamics simulations, introduced by Car and Par-rinello [112], have the ambition to model biological systems in laboratoryrelevant conditions, i.e., either in solution or in solid phase (see, e.g., [113-115]). Recently, Carloni et al. [116] applied this method to a study of the first hydrolysis step of cisplatin. They were able to reproduce satisfactorily the free energy of activation and provided a model for the transition state. Their preliminary results, which include a model of the transition state for the chelation step of the reaction between the diaqua form of cisplatin, cA-[Pt(NH3)2(H20)2]2+, and d(GpG), seem to indicate that ab initio modeling of substitution reactions on heavy-metal centers may become possible in the near future. The main drawback of Car-Parrinello calculations - their considerable computer-time cost - can be expected to abate in the next years... 

The discovery, ca. 1968, by B. Rosenberg, that the cis isomer has anti-tumor activity stimulated the synthesis and screening of over 2000 different types of complexes with different amines and anionic ligands. Platinum compounds are among the most effective medications for the treatment of advanced cancer PtCl2(NH3)2 ( Cisplatin ) is mainly used for the treatment of testicular and ovarian cancer. One of its drawbacks are the severe toxic side-effects which may be related to the facile hydrolysis of Cl ligands under physiological conditions ... 

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