Alex Sparreboom

Pharmacological effects of formulation vehicles : implications for cancer chemotherapy.

The non-ionic surfactants Cremophor® EL (CrEL; polyoxyethyleneglycerol triricinoleate 35) and polysorbate 80 (Tween® 80; polyoxy-ethylene-sorbitan-20-monooleate) are widely used as drug formulation vehicles, including for the taxane anticancer agents paclitaxel and docetaxel. A wealth of recent experimental data has indicated that both solubilisers are biologically and pharmacologically active compounds, and their use as drug formulation vehicles has been implicated in clinically important adverse effects, including acute hypersensitivity reactions and peripheral neuropathy.CrEL and Tween® 80 have also been demonstrated to influence the disposition of solubilised drugs that are administered intravenously. The overall resulting effect is a highly increased systemic drug exposure and a simultaneously decreased clearance, leading to alteration in the pharmacodynamic characteristics of the solubilised drug. Kinetic experiments revealed that this effect is primarily caused by reduced cellular uptake of the drug from large spherical micellar-like structures with a highly hydrophobic interior, which act as the principal carrier of circulating drug. Within the central blood compartment, this results in a profound alteration of drug accumulation in erythrocytes, thereby reducing the free drug fraction available for cellular partitioning and influencing drug distribution as well as elimination routes. The existence of CrEL and Tween® 80 in blood as large polar micelles has also raised additional complexities in the case of combination chemotherapy regimens with taxanes, such that the disposition of several coadministered drugs, including anthracyclines and epipodophyllotoxins, is significantly altered. In contrast to the enhancing effects of Tween® 80, addition of CrEL to the formulation of oral drug preparations seems to result in significantly diminished drug uptake and reduced circulating concentrations.The drawbacks presented by the presence of CrEL or Tween® 80 in drug formulations have instigated extensive research to develop alternative delivery forms. Currently, several strategies are in progress to develop Tween® 80- and CrEL-free formulations of docetaxel and paclitaxel, which are based on pharmaceutical (e.g. albumin nanoparticles, emulsions and liposomes), chemical (e.g. polyglutamates, analogues and prodrugs), or biological (e.g. oral drug administration) strategies. These continued investigations should eventually lead to more rational and selective chemotherapeutic treatment.

Association of variant ABCG2 and the pharmacokinetics of epidermal growth factor receptor tyrosine kinase inhibitors in cancer patients.

The purpose of the study was to determine if the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs), gefitinib and erlotinib, are substrates for the efflux transporter ABCG2, and to investigate the relevance of the ABCG2 421C>A (Q141K) polymorphism to the pharmacokinetics of gefitinib. Gefitinib and erlotinib transport in vitro was studied using HEK293 cells transfected with wild-type ABCG2 or a Q141K clone. Gefitinib pharmacokinetics was determined in 27 cancer patients. ABCG2 421C>A and ABCB1 3435C>T genotypes were determined using direct sequencing. Cells expressing wild-type ABCG2 exhibited lower intracellular accumulation of gefitinib and erlotinib at concentrations of 0.1 and 1µM, and higher efflux at 1 µM than cells lacking ABCG2 (PT genotype and gefitinib pharmacokinetics. In conclusion, gefitinib and erlotinib are ABCG2 substrates, while they inhibit ABCG2 at higher concentrations. A functional variant of ABCG2 is associated with greater gefitinib accumulation at steady-state and may be relevant to toxicity and antitumor activity of EGFR TKIs .

Clinical pharmacokinetics of docetaxel : recent developments.

Docetaxel belongs to the class of taxane antineoplastic agents that act by inducing microtubular stability and disrupting the dynamics of the microtubular network. The drug has shown a broad spectrum of antitumour activity in preclinical models as well as clinically, with responses observed in various disease types, including advanced breast cancer and non-small cell lung cancer. The pharmacokinetics and metabolism of docetaxel are extremely complex and have been the subject of intensive investigation in recent years. Docetaxel is subject to extensive metabolic conversion by the cytochrome P450 (CYP) 3A isoenzymes, which results in several pharmacologically inactive oxidation products. Elimination routes of docetaxel are also dependent on the presence of drug-transporting proteins, notably P-glycoprotein, present on the bile canalicular membrane. The various processes mediating drug elimination, either through metabolic breakdown or excretion, impact substantially on interindividual variability in drug handling. Strategies to individualise docetaxel administration schedules based on phenotypic or genotype-dependent differences in CYP3A expression are underway and may ultimately lead to more selective chemotherapeutic use of this agent.

Comparative Pharmacokinetics of Weekly and Every-Three-Weeks Docetaxel

Purpose: Weekly administration of docetaxel has demonstrated comparable efficacy together with a distinct toxicity profile with reduced myelosuppression, although pharmacokinetic data with weekly regimens are lacking. The comparative pharmacokinetics of docetaxel during weekly and once every 3 weeks (3-weekly) administration schedules were evaluated. Experimental Design: Forty-six patients received weekly docetaxel (35 mg/m2) as a 30-min infusion alone (n = 8) or in combination with irinotecan (n = 12), or in 3-weekly regimens, as a 1-h infusion at 60 mg/m2 with doxorubicin (n = 10), 75 mg/m2 alone (n = 9), or 100 mg/m2 alone (n = 7). Serial blood samples were obtained immediately before and up to 21 days after the infusion. Plasma concentrations were measured by liquid chromatography–mass spectrometry and analyzed by compartmental modeling. Results: Mean ± SD docetaxel clearance values were similar with weekly and 3-weekly schedules (25.2 ± 7.7 versus 23.7 ± 7.9 liter/h/m2); half-lives were also similar with both schedules of administration (16.5 ± 11.2 versus 17.6 ± 7.4 h). With extended plasma sampling beyond 24 h post-infusion, docetaxel clearance was 18% lower and the terminal half-life was 5-fold longer. At 35 mg/m2, the mean ± SD docetaxel concentration on day 8 was 0.00088 ± 0.00041 μg/ml (1.08 ± 0.51 nm) at 75 mg/m2, concentrations on day 8, 15, and 22 were 0.0014 ± 0.00043 μg/ml (1.79 ± 0.53 nm), 0.00067 ± 0.00025 μg/ml (0.83 ± 0.31 nm), and 0.00047 ± 0.00008 μg/ml (0.58 ± 0.099 nm), respectively. Conclusion: Docetaxel pharmacokinetics are similar for the weekly and 3-weekly regimens. Prolonged circulation of low nanomolar concentrations of docetaxel may contribute to the mechanism of action of docetaxel through suppression of microtubule dynamics and tumor angiogenesis and enhanced cell radiosensitivity in combined modality therapy.

Effect of Milk Thistle (Silybum marianum) on the Pharmacokinetics of Irinotecan

Purpose: Milk thistle (Silybum marianum) is one of the most commonly used herbal therapies, and its principal constituent silybin significantly inhibits cytochrome P450 isoform 3A4 (CYP3A4) and UDP glucuronosyltransferase isoform 1A1 (UGT1A1) in vitro. Here, we investigated whether milk thistle affects the pharmacokinetics of irinotecan, a substrate for CYP3A4 and UGT1A1, in humans. Experimental Design: Six cancer patients were treated with irinotecan (dose, 125 mg/m2) given as a 90-minute infusion once every week. Four days before the second dose, patients received 200 mg milk thistle, thrice a day, for 14 consecutive days. Pharmacokinetic studies of irinotecan and its metabolites 7-ethyl-10-hydroxycamptothecin (SN-38), 7-ethyl-10-[3,4,5-trihydroxy-pyran-2-carboxylic acid]-camptothecin (SN-38-glucuronide), and 7-ethyl-10-[4-N-(5-aminopentanoic acid)-1-piperidino]-carbonyloxycamptothecin were done during the first three irinotecan administrations. Results: Short-term (4 days) or more prolonged intake of milk thistle (12 days) had no significant effect on irinotecan clearance (mean, 31.2 versus 25.4 versus 25.6 L/h; P = 0.16). The area under the curve ratio of SN-38 and irinotecan was slightly decreased by milk thistle (2.58% versus 2.23% versus 2.17%; P = 0.047), whereas the relative extent of glucuronidation of SN-38 was similar (10.8 versus 13.5 versus 13.1; P = 0.64). Likewise, the area under the curve ratio of 7-ethyl-10-[4-N-(5-aminopentanoic acid)-1-piperidino]-carbonyloxycamptothecin and irinotecan was unaffected by milk thistle (0.332 versus 0.285 versus 0.337; P = 0.53). The maximum plasma concentrations of silybin ranged between 0.0249 and 0.257 μmol/L. Conclusions: Silybin concentrations after intake of milk thistle are too low to significantly affect the function of CYP3A4 and UGT1A1 in vivo, indicating that milk thistle is unlikely to alter the disposition of anticancer drugs metabolized by these enzymes.

Oral anticancer agents

In general medicine, oral ingestion is the most common way of drug administration, being convenient, safe and effective for most agents. In contrast, in oncology most anticancer agents are delivered by intravenous (iv) injection. This is probably due to the narrow therapeutic index of many antineoplastic drugs and the pharmacologic observation that oral administration often results in a large intra-and intersubject variability in drug exposure. However, the burden of iv administration is evident: every iv injection carries, although small, a risk of bleeding, extravasation, infection and thrombosis and requires medically qualified personnel at a hospital setting. Moreover, especially in cancer patients, repeated iv injections are hampered by the fact that a patient’s accessible vein may disappear during chemotherapy due to flebitis or thrombosis.