Probing the Platinum(IV) Prodrug Hypothesis. Are Platinum(IV) Complexes Really Prodrugs of Cisplatin?

Abstract

Amongst the metallopharmaceuticals in development, platinum(IV) complexes are unique because they are native prodrugs of clinically-relevant platinum(II) pharmacophores such as cisplatin and oxaliplatin (Fig. 3.1) These platinum(II) drugs are some of the most effective anticancer agents in clinical use and the first line treatment for many malignancies today (Fig. 3.1) (Hall et al. J Med Chem 50:3403–3411, 2007 [1]; Hall et al. Coord Chem Rev 232:49–67, 2002 [2]; Chin et al. J Med Chem 55:7571–7582, 2012 [3]). The general consensus is that these platinum(IV) prodrug complexes are themselves pharmacologically inactive and must undergo reductive elimination by endogenous reductants to release the active square-planar platinum(II) core with concomitant dissociation of the axial ligands (Fig. 3.2) (Hall et al. J Med Chem 50:3403–3411, 2007 [1]; Hall et al. Coord Chem Rev 232:49–67, 2002 [2]; Chin et al. J Med Chem 55:7571–7582, 2012 [3]). As such, the axial ligands confers unique possibilities of tuning the pharmacokinetic parameters such as lipophilicity and solubility as well as the attaching any targeting groups or synergistic co-drugs without altering the cellular mechanism of action of the innate platinum(II) pharmacophore (Hall et al. J Med Chem 50:3403–3411, 2007 [1]; Hall et al. Coord Chem Rev 232:49–67, 2002 [2]; Chin et al. J Med Chem 55:7571–7582, 2012 [3]).

Supplementary Information

Synthesis of cis-diamminechloro(4-formylbenzoate) platinum(II). Pt(NH₃)₂(Cl)₂ (50 mg, 0.167 mmol) and AgNO₃ (26.89 mg, 0.158 mmol) was stirred vigorously in H₂O (1 mL), heated at 40 °C for 2 h. The reaction suspension was filtered using a syringe filter to give a clear filtrate. Sodium formylbenzoate (31.55 mg, 0.183 mmol) was added to the filtrate and the reaction mixture was heated at 40 °C for 5 h. The resulting yellowish-white ppt was washed with H₂O (3 × 1 mL). At this stage, the pale-green crude product contained a mixture of bis-acylated cis-diamminebis(4-formylbenzoate) platinum(II) and the desired mono-acylated cis-diamminechloro(4-formylbenzoate) platinum(II). This was then dissolved in 70% MeCN/H₂O and purified by semi-preparative HPLC Figs. S3.1, S3.2.

Fig. S3.1

Semi-preparative trace of the crude Pt(II)(NH₃)₂(Cl)(carboxylbenzaldehyde). The desired product is at 16.9 min

Fig. S3.2

ESI-MS of the isolated Pt(II)(NH₃)₂(Cl)(carboxylbenzaldehyde). m/z 411.8 [M−H]⁻

Fig. S3.3

Aquation of cisplatin to form an unidentified product. HILIC chromatograms of a cisplatin in PBS (containing 139 mM Cl⁻) after leaving to stand and b cisplatin in 50 mM phosphate buffer pH 7 (without Cl⁻) after 1 h, c the DAD-UV-vis spectrum of the peak at 15.6 min showed an absence of any chromophores, excluding the possibility that it was a reduction product containing a carboxylbenzaldehyde moiety since the latter has a characteristic absorbance at 258 nm

Fig. S3.4

Left and right: HILIC (230 nm) and RPLC (214 nm) chromatograms respectively of the reduction of 2 by 2–4 mM ascorbic acid. Legend: a 4-carboxylbenzaldehyde, b 2, c dehydroascorbic acid, d ascorbic acid, e cisplatin

Fig. S3.5

Representative calibration curves. A fresh calibration curve was plotted per experiment. Top row: Calibration curves for cisplatin and 1 respectively at 230 and 305 nm. Bottom row: Calibration curve for JM118 and satraplatin respectively at 214 nm

Measurement of maximum aqueous kinetic solubility in PBS. Solid samples of 1, 2 and satraplatin were continuously added to PBS (pH 7.4) with sonication for about 20 min until the point of turbidity. The suspension was then syringe-filtered to obtain the corresponding saturated solution. Maximum solubility was then determined by ICP-OES. Experiment was repeated in duplicates (Table S3.1).

Table S3.1 Maximum solubility of 1 and 2 and satraplatin in PBS (pH 7.4). Values are reported as mean with standard error