Valerie Wiebe

Pharmacology of antineoplastic agents in pregnancy

The use of antineoplastic agents in pregnant women poses obvious risks to both the patient and the developing fetus, particularly during organogenesis. While the use of antineoplastics during pregnancy is often unavoidable, the physician may limit the risks by having a clear knowledge of the pharmacology and teratogenic potential of individual agents. Specific physiologic changes in the pregnant patient, such as enhanced renal excretion of drugs, increased or decreased hepatic function, altered gastrointestinal absorption and enterohepatic circulation, altered plasma protein binding, an increase in plasma volume (50%), and creation of a fluid filled 3rd compartment (amniotic fluid) for water soluble drugs may all significantly influence the pharmacology of antineoplastic agents. These physiological changes may effect the pregnant patients ability to absorb orally administered drugs, metabolize drugs to either active or inactive metabolites, and eliminate cytotoxically active drugs. A resulting reduction in concentration x time (C x T) for drug exposure to the maternal system may reduce the efficacy of the antineoplastic agents, while an increase in C x T may expose the patient and her fetus to undue toxicity. The timing of drug administration to gestational age is also a critical factor for some drugs. While many drugs result in adverse effects on the fetus regardless of gestational age, others appear to pose less of a threat if administered beyond the first trimester. This review addresses the pharmacology, pharmacokinetics and the teratogenic potential of individual antineoplastic agents that are commonly used in pregnant patients. The aim of this review is to help the physician select, on a patient specific basis, antineoplastic agents that avoid at least some of the fetal risk involved while maintaining efficacy in the treatment of the patient.

Tamoxifen resistance in breast cancer

Tamoxifen (TAM) resistance is the underlying cause of treatment failure in many breast cancer patients receiving TAM. The mechanism(s) involved in TAM resistance are poorly understood. A variety of mechanisms have been proposed but only limited evidence exists to substantiate them. Studies have now shown that in many patients TAM resistance is not related to the down regulation or loss of estrogen receptors (ER). Variant ER have been identified, but their significance clinically remains to be proven. Since breast cancer cells secrete several estrogen-regulated growth factors and growth inhibitors that may have autocrine or paracrine activity, altered growth factor production is another possible mechanism for TAM resistance. Tissue-specific transcription activating factors that may alter how the signal induced by TAM binding to the receptor is interpreted by the cell also require further investigation. An increase in antiestrogen binding sites (AEBS), which could effectively partition TAM and reduce its concentration at the ER has also been proposed as a potential mechanism. Pharmacologic mechanisms, such as a shift in metabolism toward the accumulation of estrogenic metabolites, are supported by recent data demonstrating metabolite E and bisphenol in tumors from TAM-resistant patients. Furthermore, a decrease in tumor TAM accumulation and an altered metabolite profile have been reported in TAM-resistant breast tumors grown in nude mice. These and other studies suggest that TAM resistance may be multifactorial in nature, but definitive identification of mechanisms that are operative in clinical TAM resistance requires further study.

Pharmacology of agents used in bone marrow transplant conditioning regimens

Abstract Disease state, donor histocompatibility, effective immunosuppression, antineoplastic activity and creation of physiological hematopoietic space are all key determinants of succesful bone marrow transplantation (BMT). Preparative BMT regimens have evolved from the use of single fraction total body irradiation (TBI) to elaborate combinations of multidrugs and fractionated TBI regimens. Many of these newly developed conditioning regiments have increased engraftment rates and curative potential. The purpose of this review is to critically examine the pharmacologic behavior of the most commonly used agents in bone marrow transplantation conditioning regimens. The known immunosuppressive, antihematopoietic and antineoplastic properties of each agent are discussed. In addition, the pharmacokinetics and toxicity profiles are evaluated. A thorough understanding of the pharmacology of agents used in BMT may aid in the design of patient specific conditioning regimens.

Targeting chemosensitizing doses of toremifene based on protein binding

SummaryToremifene is currently being evaluated as a chemosensitizing agent in doxorubicin-resistant patients. Although concentrations of >2 μM reverse resistance in vitro, target concentrations required to reverse multidrug resistance (MDR) in vivo may be highly influenced by variables such as protein binding in serum. We examined the effects of high serum concentrations on the cellular accumulation of toremifene in an MDR MDA-MB-A-1 human breast-cancer cell line. We then examined the cellular accumulation of doxorubicin at various toremifene concentrations in 5%–100% serum. We also measured the concentrations of toremifene and its major metabolites in plasma specimens obtained from two patients receiving 360 mg/day for 5 days in a phase I study. Our results show that (1) high serum concentrations decrease toremifene accumulation, (2) toremifene concentrations of ≥2.5 μm enhance doxorubicin accumulation, and (3) patients achieve plasma toremifene concentrations of 10–15 μm following doses of 360 mg/day×5 days. Our findings suggest that in vivo toremifene concentrations well above those used to reverse resistance in vitro are required to overcome the effect of high serum-protein binding.

Flow cytometry: Potential utility in monitoring drug effects in breast cancer

SummaryFlow cytometric analysis of DNA ploidy and S-phase fraction are well recognized prognostic indicators in breast cancer. The present paper deals with the widening of the applications of flow cytometry to monitoring the effectiveness of antiestrogen therapy, detecting clonal selection and emergence of drug resistance, and monitoring chemosensitizing properties of drugs. Antiestrogen activity can be studied by DNA flow cytometry to address clinical research problems such as patient-specific pharmacokinetics, dosing compliance, and acquired antiestrogen resistance. Patient plasma specimens containing various concentrations of triphenylethylenes can be monitored for drug-induced effects using cell cycle measurements and correlated toin vivo drug levels. DNA flow cytometry has also been instrumental in the study of the effects of prolonged low-dose (0.5 µM for > 100 days) tamoxifen treatment on human estrogen receptor negative MDA-MB-231 cells, where it was shown that tamoxifen may significantly alter cell cycle kinetics and tumorigenicity of these cells, selecting a new, more aggressive, and rapidly growing clone. Lastly, it has been shown that the chemosensitizing properties of another triphenylethylene antiestrogen, toremifene, on estrogen receptor negative, multidrug resistant MDA-MB-231-A1 human breast cancer cells can be studied using flow cytometric analysis. Toremifene (and its metabolites N-desmethyltoremifene and toremifene IV) are able to “resensitize” MDA-MB-231-A1 cells to vinblastine and doxorubicin, as reflected in a marked shift of cells to G2/M phase of the cell cycle. Flow cytometry is a widely available technique that might be applied clinically to monitor, at the cellular level, drug effects on tumors, including the modulators of drug resistance.

Topical toremifene: a new approach for cutaneous melanoma?

The distribution of topically applied toremifene (0.5–1 mg/day for 5 days) in the ultraviolet B (UVB)-inducedMonodelphis domestica opossum melanoma model was examined. The mean concentration of toremifene measured in the skin was 1200 nmol/g, or >500 times that detected in any other tissues (blood, brain, liver, testicles, heart, uterus, eyes). In plasma, toremifene could be detected in only one animal of six (0.04 nmol/ml). Intraperitoneal administration of 0.5 mg toremifene daily for 5 days in three female animals resulted in a mean uterus concentration of 22.9 nmol/g, or 400-fold that achieved by topical administration of 0.5 mg/day in three other femaleMonodelphis (0.05 nmol/g). The cytostatic effect of toremifene was studied in three human melanoma cell lines and three experimental cell lines derived from UVB-induced melanocytic nevi inM. domestica. Toremifene had a cytostatic effect on all cell lines (50% growth-inhibitory concentrations, 5.8–9.6 μM). Topical toremifene administration yields high local concentration with minimal systemic distribution. In addition, toremifene has a cytostatic effect at achievable concentrations in a variety of melanomatous cell lines.

Tamoxifen stimulates in vivo growth of drug-resistant estrogen receptor-negative breast cancer

An estrogen receptor-negative, multidrug-resistant MDA-MB-A1 human breast cancer cell line was grown in culture with and without a noninhibitory concentration (0.5 μM) of tamoxifen for 122 days. Tamoxifentreated and control cells were inoculated into opposite flanks of nine nude mice, where they produced measurable tumors in every case. Six of the animals were treated with tamoxifen at 500 μg/day for 22 days. Although no inhibitory nor stimulatory effect of tamoxifen was seen in vitro, tamoxifen had a clear tumor-growth-stimulating effect in mice. The most pronounced stimulatory effects were observed in the cells that had been cultured with tamoxifen. Within 3 weeks of the start of tamoxifen therapy, the cells grown in the presence of tamoxifen produced tumors with a mean size of 380 mm2, whereas the cells not pretreated with tamoxifen had tumors of 220 mm2. In contrast, in mice not receiving tamoxifen, the sizes of the tumors were 190 and 140 mm2, respectively. These preliminary results suggest that prolonged in vitro tamoxifen exposure induces cellular changes that result in tumors that are stimulated to grow faster in mice following tamoxifen treatment.

Reduced tamoxifen accumulation is not associated with stimulated growth in tamoxifen resistance

To study tamoxifen resistance-stimulated growth, 30 female ovariectomized nude mice were implanted with tamoxifen-resistant tumors and treated with 10–1000 μg/day of tamoxifen citrate subcutaneously. Tamoxifen stimulated MCF-7 tumor growth in a dose-dependent manner, with tumoral tamoxifen concentrations increasing proportionally to the dose (1–13 nmol/g), as measured by high-performance liquid chromatography (HPLC). Flow-cytometric analysis revealed that tamoxifen-resistant tumors had a different DNA content as compared with wild-type MCF-7 cells. In contrast to earlier results, these data suggest that tamoxifen resistance-stimulated growth is associated with increasing rather than decreasing tumoral tamoxifen concentrations. Furthermore, the observed ploidy changes in the tamoxifen-resistant tumors imply that a genetic basis may exist for the development of tamoxifen resistance.

A bioassay for antiestrogenic activity : potential utility in drug development and monitoring effective in vivo dosing

SummaryMonitoring effective antiestrogenic activity of the triphenylethylenes in patients with breast cancer is usually determined by the duration of response. The pharmacokinetics of toremifene and tamoxifen have been shown to be highly variable but patient specific. In the present study, we developed a method to accurately assess the antiestrogenic activity of these agents using plasma specimens, cell culture, and cell cycle measurements. Plasma specimens (4–5mls) obtained from patients receiving toremifene (360mg/day for 5 days in a phase I trial) or tamoxifen (20mg/day) were extracted and reconstituted in tissue culture media (4–5mls), and growth inhibition was determined in estrogen responsive MCF-7 cells. Additionally, plasma specimens were quantified for toremifene or tamoxifen concentrations using HPLC. Growth inhibition of plasma specimens containing either toremifene or tamoxifen and their metabolites was also examined. Cell cycle measurements were determined followingin vitro exposure with flow cytometric techniques. Our results show that a dose-response relationship exists between cell growth inhibition and cell cycle measurements for human plasma with added toremifene or tamoxifen, and also for human plasma specimens containing drug and its metabolites after treatment. Our antiestrogenic bioassay can address clinical research problems such as patient-specific pharmacokinetics, dosing compliance, and acquired antiestrogen resistance.