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Endoxifen is an active metabolite of tamoxifen, a drug used in the treatment of breast cancer. In order to be clinically effective, tamoxifen must be converted to endoxifen by cytochrome P450 2D6 (CYP2D6). A study involving single escalating oral doses was conducted in humans to evaluate the safety, tolerability, and pharmacokinetics (PK) of endoxifen. This is the first study demonstrating that single oral doses of endoxifen are safe and well tolerated and have sufficient bioavailability to reach systemically effective levels in human subjects. Furthermore, it was found that endoxifen is rapidly absorbed and systemically available and that it displays dose proportionality in peak drug concentrations in plasma (C(max)) and area under the concentration-time curve extrapolated from 0 to ∞ (AUC(0-∞)) over the dose range 0.5-4.0 mg.

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An LC-MS/MS method was developed to quantify an antisense oligonucleotide against Raf-1 expression (rafAON) in monkey and mouse plasma and in mouse tissue homogenates from animals dosed with a liposome-entrapped rafAON easy-to-use formulation (LErafAON-ETU) intended for use as an antineoplastic agent. RafAON was extracted from mouse and monkey plasma using solid-phase extraction. Tissues were homogenized and sample cleanup was achieved by protein precipitation. RafAON and the internal standard (IS) were separated on a Hamilton PRP-1 column and quantified by tandem mass spectrometry using an electrospray source in negative ion mode. The total run time was 4.0 min. The peak areas of two rafAON transitions were summed and plotted against the peak area of an IS transition to generate the standard curve. In monkey plasma the linear range was 50-10,000 ng/mL, and in mouse plasma it was 25-5000 ng/mL. The lower limit of quantification was 500 ng/mL (10 microg/g tissue) in heart, kidney, liver, lung and spleen homogenates, and the standard curve was linear up to 10,000 ng/mL. Accuracy, precision and stability were evaluated and found to be acceptable in all three matrices. The assay was used to support pharmacokinetics and tissue distribution studies of LErafAON-ETU in mice and monkeys.

Purpose: Raf proteins are key elements of growth-related cellular signaling pathways and are a component of cancer cell resistance to radiation therapy. Antisense oligonucleotides to c-raf-1 permit highly selective inhibition of the gene product and offer a strategy for sensitizing cancer cells to radiation therapy. In this dose escalation study, we evaluated the safety of combined liposomal formulation of raf antisense oligonucleotide (LErafAON) and radiation therapy in patients with advanced malignancies. Experimental Design: Patients with advanced solid tumors were treated with LErafAON in a phase I dose escalation study while receiving palliative radiation therapy. Drug-related and radiation-related toxicities were monitored. Pharmacokinetics and expression of c-raf-1 mRNA and Raf-1protein were determined in peripheral blood mononuclear cells. Results: Seventeen patients with palliative indications for radiation therapy were entered into this study. Thirteen patients received daily infusions of LErafAON and four received twice-weekly infusions. Radiation therapy was delivered in daily 300-cGy fractions over 2 weeks. Patients tolerated radiation, and no unexpected radiation-related side effects were observed. Drug-related reactions (grade z2), such as back pain, chills, dyspnea, fatigue, fever, flushing, and hypertension, were observed in most patients and were managed by premedication with corticosteroids and antihistamines. Serious adverse events occurred in five patients, including acute infusion-related symptoms, abnormal liver function tests, hypoxia, dehydration, diarrhea, esophagitis, fever, hypokalemia, pharyngitis, and tachypnea.Twelve of17 patients were evaluable for tumor response at completion of treatment; four showed partial response, four showed stable disease, and four experienced progressive disease. The intact rafAON was detected in plasma for 30 minutes to several hours. Six patients with partial response or stable disease were evaluable for c-raf-1 mRNA and/or Raf-1protein expression. Inhibition of c-raf-1mRNA was observed in three of five patients. Raf-1protein was inhibited in four of five patients. Conclusion: This is the first report of the combined modality treatment using antisense oligonucleotides with radiation therapy in patients with advanced cancer. A dose of 2.0 mg/kg of LErafAON administered twice weekly is tolerated with premedication and does not enhance radiation toxicity in patients. The observation of dose-dependent, infusion-related reactions has led to further modification of the liposomal composition for use in future clinical trials.

Endoxifen is the key active metabolite of tamoxifen, a widely used breast cancer drug. Orally administered tamoxifen, is extensively metabolized by cytochrome P450 (CYP) enzymes, namely CYP3A4 and CYP2D6, into active metabolites, especially endoxifen. Due to genetic polymorphism of CYP2D6, significant numbers of women metabolize tamoxifen to varying degree and may not receive the optimal benefit from tamoxifen treatment. We show that oral administration of endoxifen achieved the optimally effective systemic levels reliably, which may eliminate variability associated with tamoxifen metabolism that leads to unpredictability in efficacy. Furthermore, use of endoxifen may avoid a potential serious drug interaction found between tamoxifen and commonly used selective serotonin reuptake inhibitors, antidepressants. Endoxifen was active in inhibiting the growth of various breast tumor cell lines in NCI 60-Cell Line Screen. Orally administered endoxifen is rapidly absorbed and systemically available when tested in female rats. The endoxifen-treated rats showed 787% higher exposure (AUC(0-infinity)) and 1,500% higher concentration (C (max)) levels of endoxifen when compared with tamoxifen. Oral endoxifen administration once a day for 28 consecutive days at dosages 2, 4, and 8 mg/kg proved safe and resulted in progressive inhibition of the growth of the human mammary tumor xenografts in female mice. This is the first ever in vivo report on endoxifen as a potentially new therapeutic agent for breast cancer.

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