Effects of amino acids on the transport and cytotoxicity of melphalan by human bone marrow cells and human tumor cells

Clinical pharmacokinetics of oral drugs in the treatment of multiple myeloma

Treatment of myeloma is a long-term treatment mainly based on all-oral combinations of drugs. Because oral drugs have a more complex pharmacokinetics compared with IV treatments, an appropriate knowledge of the factors that may alter their systemic exposure is of particular clinical relevance. Both drug-drug interactions, food-effect, and dose-adaptation in renal and hepatic impairment may influence the systemic drug levels with a potential impact on drug efficacy or safety. Moreover, a better control of drug exposure may improve the side effect profiles of these treatments with a favourable impact on patient compliance. Furthermore, as long-term treatments, these drugs may also alter the systemic exposure of coadministered medications in these rather old patients. The aim of this review was to identify the factors modifying the systemic exposure of oral drugs used in myeloma by focusing on the pharmacokinetic drug-drug interactions and the effects of renal and hepatic impairment and of food impact.

Pharmacokinetics & drug resistance of melphalan in regional chemotherapy: ILP versus ILI

Two forms of regional chemotherapy for the treatment of advanced melanoma or sarcoma of the extremity are isolated limb perfusion (ILP) and the more recently described isolated limb infusion (ILI). Melphalan is the most commonly employed agent in both ILP and ILI, although it is often used in conjunction with other cytotoxic and/or biologic therapies. While ILP and ILI are far more effective for the treatment of extremity disease than is systemic therapy, there is still significant room for improvement in outcomes, from the standpoint of both response rate and toxicity. An understanding of the pharmacokinetics of regional chemotherapy would allow for the prediction of tumor response and toxicity and therefore patient outcomes. In addition, elucidating the mechanisms of drug resistance would lead to opportunities to develop effective chemo-modulators that enhance the effectiveness of ILP and ILI. This paper reviews progress in these two key areas of active investigation.

Nitrogen mustard-DNA interaction in melphalan-resistant mammary carcinoma cells with elevated intracellular glutathione and glutathione-S-transferase activity

We examined the relationship between intracellular levels of glutathione (GSH), glutathione-S-transferase (GST) activity, and the kinetics of DNA cross-links induced by the bifunctional alkylating drugs melphalan (MLN), chlorambucil (CLB), and mechlorethamine (HN2) in a rat mammary carcinoma cell line (WT) and in a subline selected in vitro for primary resistance to MLN (MLNr, 16-fold resistance). MLNr cells exhibit a 2-fold increase in intracellular GSH concentration and an approximately 5-fold increase in GST activity as compared with the parent cells. They are cross-resistant to a variety of drugs, including CLB (6-fold) and HN2 (14-fold). Treatment of WT cells with 30 microM MLN or CLB induced a significant accumulation of DNA-DNA cross-links for up to 8 h, which decreased over a 24-h period. In MLNr cells, no significant cross-link formation was induced by either MLN of CLB at any time between 0 and 24 h. Doses of up to 100 microM MLN failed to induce cross-links in MLNr cells. Formation of cross-links was observed immediately after treatment with HN2 in both cell lines and was followed by a subsequent decrease during a 24-h incubation in drug-free medium. At an equimolar concentration (30 microM), the numbers of HN2-induced cross-links were significantly lower in MLNr cells than in WT cells. However, treatment of MLNr cells with 60 microM HN2 resulted in cross-link levels similar to those obtained using 30 microM HN2 in WT cells. The 35% decrease in MLN accumulation observed in MLNr cells could not entirely explain the absence of cross-links, since thin-layer chromatographic analysis demonstrated that both cell lines accumulate a significant amount of MLN and metabolize it to the same extent. Significant amounts of MLN were also detected in nuclei isolated from WT and MLNr cells that had been treated with 30 microM [14C]-MLN. Intracellular depletion of GSH by a nontoxic concentration of L-buthionine-(S, R)-sulfoximine (BSO, 100 microM; about 70% GSH depletion) significantly sensitized MLNr cells to MLN and increased cross-link formation. A nontoxic concentration (50 microM) of ethacrynic acid (EA, an inhibitor of GST showing some specificity for Yc/Yp subunits) also sensitized MLNr cells to MLN and increased cross-link formation. Our data demonstrate that both EA and BSO are effective modulators of nitrogen mustard cytotoxicity in tumor cells resistant to alkylating drugs.(ABSTRACT TRUNCATED AT 400 WORDS)

Tyrosine transport in a human melanoma cell line as a basis for selective transport of cytotoxic analogues

Tyrosine is an essential amino acid for the initial step of melanin synthesis, yet little is known concerning its transport in melanocytes. As an important first step in the development of new anti-melanoma agents based upon chemical and pharmacological modifications of melanin synthesis, the present study characterized the transport mechanism of tyrosine in vitro using the human melanoma cell line SK-MEL 23. Several tyrosine transport systems may be involved in melanocytes: systems L and T, which transport neutral amino acids with branched or aromatic side chains, and systems A and ASC, which transport neutral amino acids with smaller side chains. In order to determine which system or combination of systems is involved in tyrosine transport in melanoma cells, studies of kinetics, Na(+)-dependence and competitive inhibition were undertaken. The Km and Vmax. for the Na(+)-independent transport system were found to be 0.164 +/- 0.016 mM and 21.6 +/- 1.1 nmol/min per mg of protein respectively. This transport was preferentially inhibited by the system L specific analogue, 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid, the system T substrate tryptophan, and the sulphur homologue of tyrosine, 4-S-cysteinylphenol. Sequential addition of these inhibitors at increasing concentrations indicated that they inhibit the same transporter. Our results suggest that tyrosine transport in SK-MEL 23 melanoma cells is similar to system L transport previously characterized in other cell types. This one transport system appears to supply all the tyrosine required for both cell growth and melanin synthesis. The transport system may be subject to manipulation by melanogenic stimulating factors, making the transport of cytotoxic tyrosine analogues an important area for further study.

The mechanism of acquired resistance to cisplatin by a human ovarian cancer cell line

The present study was designed to elucidate the mechanism of resistance to cisplatin. A cisplatin-resistant cell line (KFr) was established from KF cells derived from human serous cystadenocarcinoma of the ovary. The DNA histogram revealed an increase of S-phase cells and a decrease of G1-phase cells in cultured KFr cells, compared to that in cultured KF cells. Although the cisplatin content in the KF cells incubated with cisplatin at 10 micrograms/ml increased in a time-dependent manner, that in the KFr cells remained unchanged during the experimental period. When 0.5 mg of cisplatin was administered ip to nude mice with KF or KFr tumor, the cisplatin content in the KFr tumor was significantly lower than that in the KF tumor. The KFr cells showed a cross-resistance to L-phenylalanine mustard, while no cross-resistance to vincristine or 5-fluorouracil was observed. These findings suggest that the mechanism of cisplatin resistance in the KFr cells involves a decrease of cisplatin accumulation in the tumor cells.

Effect of L-leucine on oral melphalan kinetics in patients

Melphalan uptake in the intestine has recently been shown to be an energy-dependent process which is affected by metabolic inhibitors. It is therefore theoretically possible that amino acids in food could reduce melphalan absorption by competing for uptake at the sites of absorption in the intestine. Since L-leucine has been shown to be the most potent inhibitor of melphalan transport into cells in vitro, this amino acid was chosen for the present study in patients. Oral melphalan (4.5 +/- 0.5 mg/m2) was given to ten fasting patients with and without a 2-g oral dose of L-leucine on separate randomized occasions at least 1 week apart. Melphalan plasma levels were measured by high-performance liquid chromatography (HPLC) for 5-h after dosing. L-Leucine plasma levels were measured by HPLC before and at 1 h after dosing. The area under the curve for melphalan was lower in seven of the patients after L-leucine. Plasma L-leucine levels 1 h after melphalan administration were 15.4 +/- 3.7 micrograms/ml fasting and 35.4 +/- 5.2 micrograms/ml after L-leucine. The results indicate that L-leucine can reduce plasma melphalan levels in some patients, probably through a reduction in absorption of the drug from the gastrointestinal tract. However, the effect, like that of food, is highly variable.