Platinum(IV)-Loaded Degraded Glycol Chitosan as Efficient Platinum(IV) Drug Delivery Platform

Abstract

A new class of anticancer prodrugs was designed by combining the cytotoxicity of platinum(IV) complexes and the drug carrier properties of glycol chitosan polymers: Unsymmetrically carboxylated platinum(IV) analogues of cisplatin, carboplatin and oxaliplatin, namely (OC-6-44)-acetatodiammine(3-carboxypropanoato)dichloridoplatinum(IV), (OC-6-44)-acetaodiammine(3-carboxypropanoato)(cyclobutane-1,1-dicarboxylato)platinum(IV) and (OC-6-44)-acetato(3-carboxypropanoato)(1R,2R-cyclohexane-1,2-diamine)oxalatoplatinum(IV) were synthesised and conjugated via amide bonding to degraded glycol chitosan (dGC) polymers with different chain lengths (5, 10, 18 kDa). The 15 conjugates were investigated with ¹H and ¹⁹⁵Pt NMR spectroscopy, and average amounts of platinum(IV) units per dGC polymer molecule with ICP-MS, revealing a range of 1.3-22.8 platinum(IV) units per dGC molecule. Cytotoxicity was tested with MTT assays in the cancer cell lines A549, CH1/PA-1, SW480 (human) and 4T1 (murine). IC₅₀ values in the low micromolar to nanomolar range were obtained, and higher antiproliferative activity (up to 72 times) was detected with dGC-platinum(IV) conjugates in comparison to platinum(IV) counterparts. The highest cytotoxicity (IC₅₀ of 0.036 ± 0.005 µM) was determined in CH1/PA-1 ovarian teratocarcinoma cells with a cisplatin(IV)-dGC conjugate, which is hence 33 times more potent than the corresponding platinum(IV) complex and twice more potent than cisplatin. Biodistribution studies of an oxaliplatin(IV)-dGC conjugate in non-tumour-bearing Balb/C mice showed an increased accumulation in the lung compared to the unloaded oxaliplatin(IV) analogue, arguing for further activity studies.

Keywords: anticancer; drug delivery; glycol chitosan; platinum(IV) complexes.

Scheme 1

Overview of different application areas of chitosan and its derivatives (not exhaustive) [22].

Scheme 2

Overview of process steps and delivery concept of platinum(IV)–dGC conjugates [26,32]. In the presented work, the synthesis of dGC polymers started with degradation of glycol chitosan, whereas platinum(IV) complexes were obtained by modification and oxidation of K₂PtCl₄. Additionally, in vivo studies were performed in non-tumour-bearing mice.

Scheme 3

Synthetic pathway for platinum(IV) complexes 1–6 with mixed axial ligands and a free carboxylic acid group to enable the introduction of drug delivery platforms.

Scheme 4

Time-dependent acidic degradation of glycol chitosan results in dGC polymers with different molecular weights (MW) P1–P3. The remaining degree of acetylation was determined with ¹H NMR spectroscopy and was detected as about 1% for P1, 2% for P2, and 4% for P3.

Scheme 5

Scheme of CDI coupling reaction of the unsymmetric platinum(IV) complexes and dGC polymers, yielding 20 conjugates with a platinum(IV) to dGC polymer ratio between 1.3 and 22.8.

Figure 1

Structure–activity relationship of the studied platinum(IV) complexes, dGC polymers and conjugates. The y-axis represents IC₅₀ values (means ± standard deviations) in all three cell lines on a logarithmic scale. The arrows indicate IC₅₀ values higher than the indicated value.

Figure 2

Correlation between IC₅₀ values and platinum(IV) loading of oxaliplatin derivatives (C6–C12). The y-axis represents IC₅₀ values (means ± standard deviations) in all three cell lines on a logarithmic scale; the x-axis is related to the platinum(IV) units per polymer.

Figure 3

Overview of the accumulation of platinum in different organs, measured with ICP-MS, as result of the biodistribution studies for platinum(IV) complex 6 and conjugate V3 after 1 h and 24 h after intravenous administration. Additionally, significance of the increase and decrease in the platinum amount between 1 h and 24 h of complex 6 (upper figure) and conjugate V3 (lower figure) was further examined via unpaired t-test with Welch’s correction, with the following abbreviations: ns = not significant; * p < 0.05; ** p < 0.01; *** p < 0.001; **** p < 0.0001.

Figure 3

Overview of the accumulation of platinum in different organs, measured with ICP-MS, as result of the biodistribution studies for platinum(IV) complex 6 and conjugate V3 after 1 h and 24 h after intravenous administration. Additionally, significance of the increase and decrease in the platinum amount between 1 h and 24 h of complex 6 (upper figure) and conjugate V3 (lower figure) was further examined via unpaired t-test with Welch’s correction, with the following abbreviations: ns = not significant; * p < 0.05; ** p < 0.01; *** p < 0.001; **** p < 0.0001.