A pH responsive complexation-based drug delivery system for oxaliplatin

Abstract

A responsive drug delivery system (DDS) for oxaliplatin (OX) has been designed with a view to overcoming several drawbacks associated with this anticancer agent, including fast degradation/deactivation in the blood stream, lack of tumor selectivity, and low bioavailability. The present approach is based on the direct host-guest encapsulation of OX by a pH-responsive receptor, carboxylatopillar[6]arene (CP6A). The binding affinities of CP6A for OX were found to be pH-sensitive at biologically relevant pH. For example, the association constant (K_a) at pH 7.4 [K_a = (1.02 ± 0.05) × 10⁴ M^-1] is 24 times larger than that at pH 5.4 [K_a = (4.21 ± 0.06) × 10² M^-1]. Encapsulation of OX within the CP6A cavity did not affect its in vitro cytotoxicity as inferred from comparison studies carried out in several cancer cells (e.g., the HepG-2, MCF-7, and A549 cell lines). On the other hand, complexation by CP6A serves to increase the inherent stability of OX in plasma by 2.8-fold over a 24 h incubation period. The formation of a CP6A⊃OX host-guest complex served to enhance in a statistically significant way the ability of OX to inhibit the regrowth of sarcoma 180 (S180) tumors in Kunming (KM) mice xenografts. The improved anticancer activity observed in vivo for CP6A⊃OX is attributed to the combined effects of enhanced stability of the host-guest complex and the pH-responsive release of OX. Specifically, it is proposed that OX is protected as the result of complex formation and then released effectively in the acidic tumor environment.

Scheme 1. The structures of CP5A, CP6A, and OX.

Fig. 1. ¹H NMR spectra (400 MHz, 293 K D₂O) of (A) OX (4.9 mM), (B) OX (4.8 mM) + CP6A (5.0 mM), and (C) CP6A (5.0 mM).

Fig. 2. In vitro and in vivo stability of OX in the absence and presence of 1.0 equiv. of CP6A. (A) In vitro stability. Residual percentage of OX in plasma in the absence and presence of 1.0 equiv. of CP6A as determined by RP-HPLC. The initial concentration of OX is 940 μM. (B) and (C) In vivo stability experiments. Pt content in red blood cells (B) and in plasma (C) at time intervals of 15 min and 1 h after injection of OX or a 1 : 1 mixture of CP6A and OX into rats as determined by ICP-MS. The dose of OX was 15 mg kg^–1 and the initial concentration of the solutions used for injection was 4.72 mM.

Fig. 3. Antiproliferative activity of free OX and a 1 : 1 mixture of OX and CP6A as seen in (A) MCF-7, (B) HepG-2, and (C) A549 cells as determined using CCK-8. Cells were treated with either OX or OX and CP6A for 72 h followed by incubation for 24 h in fresh medium.

Fig. 4. In vivo antitumor experiments. Change in body weight observed for S180 xenograft mice (A), normalized tumor weights after treatment (B), and pictures of the tumors (C) excised from S180 xenograft mice treated with saline (control), free OX, and a 1 : 1 mixture of OX and CP6A at an OX dose of 15 or 35 mg kg^–1 as indicated. ***p < 0.001.

Fig. 5. Schematic illustration of the DDS effect expected to operative in the case of a 1 : 1 mixture of OX and CP6A.