Beta-tubulin and P-glycoprotein: major determinants of vincristine accumulation in B-CLL cells

Physiologically Based Pharmacokinetic Models for Adults and Children Reveal a Role of Intracellular Tubulin Binding in Vincristine Disposition

Vincristine is a cytotoxic chemotherapeutic agent used as first‐line therapy for pediatric acute lymphocytic leukemia. It is cleared by hepatic oxidative metabolism by CYP3A4 and CYP3A5 and via hepatic (biliary) efflux mediated by P‐glycoprotein (P‐gp) transporter. Bottom‐up physiologically based pharmacokinetic (PBPK) models were developed to predict vincristine disposition in pediatric and adult populations. The models incorporated physicochemical properties, metabolism by CYP3A4/5, efflux by P‐gp, and intracellular binding to β‐tubulin. The adult and pediatric PBPK models predicted pharmacokinetics (PK) within twofold of the observed PK parameters (area under the curve, terminal half‐life, volume of distribution, and clearance). Simulating a higher hypothetical (4.9‐fold) pediatric expression of β‐tubulin relative to adult improved predictions of vincristine PKs. To our knowledge, this is the first time that intracellular binding has been incorporated into a pediatric PBPK model. Utilizing this PBPK modeling approach, safe and effective doses of vincristine could be predicted.

High expression of Bcl-2 predicts poor outcome in diffuse large B-cell lymphoma patients with low international prognostic index receiving R-CHOP chemotherapy

The prognostic significance of Bcl-2, Bcl-6, p53, topoisomerase II, and β-tubulin expression was evaluated in diffuse large B-cell lymphoma (DLBCL) patients treated with cyclophosphamide, doxorubicin, vincristine, prednisolone, and rituximab. Eight-year progression-free survival (PFS, P = 0.006) and overall survival (OS, P = 0.001) of patients with high Bcl-2 expression were significantly inferior to those of patients with low expression without prognostic significance of Bcl-6, p53, topoisomerase II, and β-tubulin expression. High expression of Bcl-2 was associated with poor PFS (P = 0.045) and OS (P = 0.004) only in patients with low international prognostic index (IPI). In multivariate analysis, high expression of Bcl-2 was a significant independent prognostic factor of poor PFS (P = 0.026) and OS (P = 0.007) along with high IPI. In conclusion, the expression of Bcl-2 may be a useful prognostic factor, especially in DLBCL patients with low IPI.

Is P-glycoprotein (ABCB1) a phase 0 or a phase 3 colchicine transporter depending on colchicine exposure conditions?

This study investigates the P-glycoprotein (Pgp)-mediated transport of its substrates in accumulation or efflux modes under steady-state conditions. The kinetics of colchicine uptake and efflux, a substrate of both Pgp and intracellular tubulin, were studied in HL60 and HL60/DNR cells; HL60/DNR cells contain 25 times more Pgp than do HL60 cells. HL60/DNR cells in a medium containing 6.25 nM colchicine, which mimics therapeutic conditions, reached steady-state twice as rapidly as did HL60 cells, and accumulated 24-times less colchicine than did HL60 cells. The Pgp inhibitor GF120918, increased colchicine uptake by HL60 cells 1.2-fold and that of HL60/DNR cells 17-fold, while it had no effect on colchicine efflux from either cell line that had been incubated with colchicine for 24 h. Colchicine kinetics fitted well a two closed-compartment model, showing that the low intracellular accumulation of colchicine in HL60/DNR cells resulted from a 11-fold decrease in colchicine uptake and a 2.3-fold increase in colchicine efflux, that could be attributed to Pgp-mediated efflux activity in HL60/DNR cells. Intracellular colchicine was mainly and similarly distributed in the cytosol in both cell lines. These data demonstrate that the kinetics of the intracellular colchicine accumulation depend on the density of Pgp and that Pgp is more a phase 0 (preventing cellular uptake) than a phase 3 (effluxing intracellular substrate) transporter under steady-state conditions, although the situation is reversed after a short incubation time (30 min), when intracellular free colchicine concentration is probably high enough for it to be removed from the cell by Pgp.

Nocodazole, a microtubule depolymerising agent, induces apoptosis of chronic lymphocytic leukaemia cells associated with changes in Bcl-2 phosphorylation and expression

Microtubule active drugs are used in the treatment of malignancies and their mechanism of action in cycling cells is to produce mitotic arrest followed by apoptosis. In this study, we investigate in detail the specificity and mechanism by which a microtubule de-polymerising agent, nocodazole, induces apoptosis in non-cyclingm, i.e. G(0)/G(1), chronic lymphocytic leukaemia (CLL) B-cells. The majority of cases of CLL are sensitive (IC(50)<or=16 microM) but normal peripheral blood B-cells, which are also in G(0)/G(1), are resistant to the maximum in vitro concentration of this agent. Taxol, a microtubule stabilising drug does not kill CLL cells suggesting a specific effect of nocodazole. The mechanism of apoptosis involves mitochondrial membrane depolarisation, activation of caspases and cleavage of PARP. Nocodazole causes two patterns of change to Bcl-2 expression. In one there is increase in expression of the serine-70 phosphorylated form of Bcl-2 and in the other total Bcl-2 expression is reduced. Collectively the data shows that sensitivity to nocodazole-induced apoptosis is a feature of chronic lymphocytic leukaemia and suggests that newer microtubule active agents be systematically investigated for their effectiveness in this condition.

Autoantibodies to tubulin are specifically associated with the young age onset of the nasopharyngeal carcinoma

By far the highest incidence of the Epstein-Barr virus (EBV)-associated nasopharyngeal carcinomas in young subjects was found in North Africa, a region of intermediate risk for adults. We used the immunofluorescence analysis and ELISA to characterize the presence of IgG autoantibodies to the cytoskeleton and nuclear proteins in sera of 82 Tunisian patients with primary nasopharyngeal carcinoma and those of 82 healthy subjects. To assess the specificity of the sera autoreactivity, inhibition tests were carried out using free autoantigens. Analysis of sera autoreactivity in patients with nasopharyngeal carcinoma and in control subjects showed that 23% of the patients had serum reactivity against more than 1 autoantigen tested compared to 1.2% in the control group (p = 10(-4)). The most frequent auto reactivity in patient's sera was found with tubulin and nuclear proteins (19.5% and 22% respectively vs. 6.1% and 1.2% in controls). The IgG auto reactivity inhibition studies indicate that all autoantigens, except native DNA, showed low values of IC(50) (concentration of antigen causing 50% inhibition of the antibody binding) reflecting the high affinity of these IgG autoantibodies. When patients and controls were stratified according to their age, IgG autoantibodies to tubulin were found specifically associated with the young age onset of the nasopharyngeal carcinoma (age under 25 years). IgG auto reactivity comparison before and after cancer therapy showed that only anti-tubulin reactivity was significantly affected by treatment. Our results demonstrate that the autoantibodies to the cytoskeleton and nuclear proteins are associated with the nasopharyngeal carcinoma in Tunisians. The anti-tubulin IgG autoantibodies may represent a serologic marker for the nasopharyngeal carcinoma in children and adolescents Tunisians.