Pharmacokinetic Analysis and in Vivo Antitumor Efficacy of Taccalonolides AF and AJ

Abstract

The taccalonolides are microtubule stabilizers that covalently bind tubulin and circumvent clinically relevant forms of resistance to other drugs of this class. Efforts are under way to identify a taccalonolide with optimal properties for clinical development. The structurally similar taccalonolides AF and AJ have comparable microtubule-stabilizing activities in vitro, but taccalonolide AF has excellent in vivo antitumor efficacy when administered systemically, while taccalonolide AJ does not elicit this activity even at maximum tolerated dose. The hypothesis that pharmacokinetic differences underlie the differential efficacies of taccalonolides AF and AJ was tested. The effects of serum on their in vivo potency, metabolism by human liver microsomes and in vivo pharmacokinetic properties were evaluated. Taccalonolides AF and AJ were found to have elimination half-lives of 44 and 8.1 min, respectively. Furthermore, taccalonolide AJ was found to have excellent and highly persistent antitumor efficacy when administered directly to the tumor, suggesting that the lack of antitumor efficacy seen with systemic administration of AJ is likely due to its short half-life in vivo. These results help define why some, but not all, taccalonolides inhibit the growth of tumors at systemically tolerable doses and prompt studies to further improve their pharmacokinetic profile and antitumor efficacy.

Figure 1

Structure of taccalonolides AF and AJ

Figure. 2

Plasma concentration-time curves for the taccalonolides. The concentration of taccalonolide AF and AJ in mouse plasma was calculated over time after IV tail vein injection of 300 µg of drug. Individual points represent data from 3 – 5 mice ± SEM.

Figure 3

The effect of serum concentration on in vitro potency. The antiproliferative and cytotoxic potencies of paclitaxel, taccalonolide AF and taccalonolide AJ were evaluated in the presence of 0.1% (green), 1% (red) or 10% (blue) serum. A dashed line at 0% cell growth denotes the density of cells at the time of drug addition, such that values above and below this line indicate antiproliferative and cytotoxic activities, respectively. Individual points represent the mean ± SEM for 3 independent experiments with each concentration tested in triplicate.

Figure 4

Antitumor efficacy of intratumorally administered taccalonolides. (A) The average tumor volume ± SD of SCC-4 xenografts in mice treated with a total dose of 80 µg AF, 40 or 80 µg AJ, or vehicle with administration split between days 0 and 3. A two-way ANOVA with Tukey’s post-hoc for multiple comparisons over the duration of the trial was performed and statistical significance between vehicle and drug-treated tumors is noted on day 17; *p < 0.05; ****p < 0.0001. (B) Percentage of tumors smaller than 1,500 mm³ over time.