Ultra-sensitive speciation analysis of inorganic platinum-chloride complexes in platinum-based drugs by HPLC-ICP-MS

Platinum-based drugs are widely used in chemotherapy against a broad range of cancer types. Cisplatin (1978), carboplatin (1989) and oxaliplatin (2002) are three members of the platinum anticancer drug family approved in many counties.  These compounds have poor oral bioavailability and are therefore administered by injection, a method in which serious safety problems can be caused by impurities.

Impurities can be introduced during the synthesis process of materials by reagents, intermediates, by-products, and isomers. In general, potassium chloroplatinate (K₂PtCl₄) or potassium hexachloroplatinate (K₂PtCl₆) are raw materials for the synthesis of platinum-based drugs. Therefore, the residues of K₂PtCl₄ and K₂PtCl₆ in the final platinum-based drugs should be strictly controlled as part of the quality control. While the rule makers in the different countries have established limits for individual impurities or total impurities, they do not provide any methods for the quantification of K₂PtCl₄ and K₂PtCl₆. The European Pharmacopoeia (EP 8.0) has specified that the content of individual impurities in platinum-based drugs should not exceed 0.1%.

Figure: Structures of the platinum compounds discussed here

A group of Chinese researchers aimed at developing such method which can provide a reference basis for the scientific evaluation of the safety of platinum-based drugs. Typically, Pt speciation analysis is performed by using a separation method such as GC, CE, HILIC or HPLC coupled with a sensitive detection technique such as ICP-MS. The hyphenated technique HPLC-ICP-MS is particularly attractive for liquid samples, due the ease of sample preparation, the simple interface and the many separation modes available.

One of the challenges to quantify the Pt-impurities in Pt-based drugs is the low concentration of the impurities that has to be detected in presence of very high concentration of the Pt-drug contaminating and saturating the detection system. The authors therefore used an on-line switching technology meant to bypass the Pt-drug elution peak to waste instead of feeding it to the ICP-MS. In order to program the switching device, the authors investigated the elution sequence of the different compounds by injecting a mixed standard.  Different columns were tested and the Hamilton PRP-X100 analytical column was selected as the optimum. In order to avoid any long time stability problems by introducing high concentrations of buffer systems, a low concentration of HCl solution (2 mmol/L) was used as mobile phase for isocratic elution. Using these HPLC parameters the 5 Pt species were separated in less than 5 min. in the order of cisplatin (0.2-0.5 min), oxaliplatin (0.6-0.8 Min), K₂PtCl₆ (1.5-2.0 min), K₂PtCl₄ (2.1-2.7 min) and carboplatin (3.4-4.2 min). Therefore, in order to bypass the Pt-drugs, the solution flowing from the column was discharged to waste in the range of 0.2-1 min and 3-5min. This allowed for the sensitive detection of the two impurities, with limits of quantitation of 0.7 µg/L for K₂PtCl₆ and 1.0 µg/l for K₂PtCl₄. The accuracy of the method was checked by spiking real samples with the target analytes. Spike recoveries between 94% and 103% demonstrated satisfactory accuracy.

The original study

Y. Nygren, P. Hemström, C. Åstot. Hydrophilic interaction liquid chromatography (HILIC) coupled to inductively coupled plasma mass spectrometry (ICPMS) utilizing a mobile phase with a low-volatile organic modifier for the determination of cisplatin, and its monohydrolyzed metabolite. J. Anal. At. Spectrom., 23 (2008) 948-954. DOI: 10.1039/b716093c

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