Comparative Preclinical Pharmacokinetic and Metabolic Studies of the Combretastatin Prodrugs Combretastatin A4 Phosphate and A1 Phosphate

Abstract

Purpose: Combretastatin A4 phosphate (CA4P) and its structural analog, combretastatin A1 phosphate (CA1P), are soluble prodrugs capable of interacting with tubulin and causing rapid vascular shutdown within tumors. CA4P has completed Phase I clinical trials, but recent preclinical studies have shown that CA1P displays a greater antitumor effect than the combretastatin A4 (CA4) analog at equal doses. The aim of this study, therefore, is to compare pharmacokinetics and metabolism of the two compounds to determine whether pharmacokinetics plays a role in their differential activity.Experimental Design: NMRI mice bearing MAC29 tumors received injection with either CA4P or CA1P at a therapeutic dose of 150 mg⅐kg ؊1 , and profiles of both compounds and their metabolites analyzed by a sensitive and specific liquid chromatography/mass spectroscopy method.Results: The metabolic profile of both compounds is complex, with up to 14 metabolites being detected for combretastatin A1 (CA1) in the plasma. Many of these metabolites have been identified by liquid chromatography/mass spectroscopy. Initial studies, however, focused on the active components CA4 and CA1, where plasma and tumor areas under the curve were 18.4 and 60.1 g⅐h⅐ml ؊1 for CA4, and 10.4 and 13.1 g⅐h⅐ml ؊1 for CA1, respectively. In vitro metabolic comparisons of the two compounds strongly suggest that CA1 is metabolized to a more reactive species than the CA4.Conclusions: Although in vitro studies suggest that variable rates of tumor-specific prodrug dephosphorylation may explain these differences in pharmacokinetics profiles, the improved antitumor activity and altered pharmacokinetic profile of CA1 may be due to the formation of a more reactive metabolite.

Vascular effects dominate solid tumor response to treatment with combretastatin A‐4‐phosphate

“…These drug exposures are relevant for in vivo studies, where mouse plasma levels of the active CA4P metabolite CA4 were reported in the region of 0.5-2.8 lM, within 90 min of administering 100 mg/kg CA4P. 29,43 Our data confirmed several previous studies [24][25][26][27][28] showing that the mode of direct tumor cell death was anti-proliferative, leading to apoptosis, and that resistance to CA4P was indeed maintained in vivo, since markers of apoptosis, namely DNA fragmentation and cleaved caspase-3 expression were significantly greater in SW1222…”

“…Mitotic-arrested cells subsequently undergo mitotic catastrophe and apoptosis through caspase-dependent and -independent mechanisms. In vivo, the plasma half-life of the active CA4 is short 11,29 and it is difficult to predict whether drug exposures are great enough for significant direct cytotoxic effects to occur under these conditions, especially as there may be a degree of drug trapping within tumor tissue, as a consequence of vascular collapse. To maximize efficacy and achieve local tumor control, it is necessary to combine vascular disrupting agents with other therapeutic approaches such as conventional chemotherapy or radiotherapy.…”

Phase I Clinical and Pharmacokinetic Evaluation of the Vascular-Disrupting Agent OXi4503 in Patients with Advanced Solid Tumors

“…It is a synthetic, phosphorylated prodrug of combretastatin A1 (OXi4500), a naturally occurring derivative from the bark of the South African bush willow tree, Combretum caffrum, that reversibly binds to the b-subunit at the colchicine-binding site of tubulin to inhibit microtubule assembly (3,4). It is metabolized to a reactive orthoquinone species that is also assumed to be directly cytotoxic in tumor cells (5,6) because of the production of a quinone metabolite that could bind to nucleic acids and also produces free radicals leading to the enhancement of oxidative stress (7).…”

1,4-Diamino-2,3-dicyano-1,4-bis(methylthio)butadiene (U0126) Enhances the Cytotoxicity of Combretastatin A4 Independently of Mitogen-Activated Protein Kinase Kinase

“…It has been suggested that dephosphorylation of CA4P into its active form, CA4, occurs relatively faster (Tozer et al, 1999), whereas glucuronidation of CA4 is much slower in tumor tissues than in plasma (Rustin et al, 2003;Kirwan et al, 2004). This makes CA4P preferentially target tumors.…”