Abstract P1-08-03: Identification and characterization of a novel endoxifen substrate, PKCβ1, and its interaction with the estrogen receptor

Background: The primary mechanism by which tamoxifen (Tam) and its metabolites exert their biologic effects is through estrogen receptor (ER) binding and inhibition of ER signaling. We and others demonstrated that endoxifen (Endx) has greater antitumor activity in vitro and in vivo compared to Tam and the first-in-human Endx phase I study demonstrated its antitumor activity in patients with prior progression on Tam (Goetz SABC 2015). PKCs are a family of serine/threonine-specific protein kinases that regulate signaling pathways involved in cell proliferation and tumorigenic transformation. Our prior protein docking studies suggested endoxifen may be a substrate for PKCs. Here we report the effects of Tam and Endx on PKCβ1 binding, kinase activity, as well as interactions between PKCβ1 and ERα.

Methods: Surface Plasmon Resonance (SPR, Biacore T200, GE Healthcare) was used to evaluate binding of Tam, N-desmethyl Tam (NDMT), 4-HT, and Endx to PKCβ1 and PKCβ2. The effects of Tam and Endx on PKCβ1 kinase activity were determined. Proliferation and colony formation in MCF7 parental and PKCβ1 overexpressing cells were evaluated. siRNA silencing was used to knockdown PKCβ1 expression in the following cells: MCF7 aromatase expressing cells that were either sensitive (MCF7/AC1) or resistant to letrozole (MCF7/AC1 L-resistant); T47D; and MDA-MB-361. Coimmunoprecipitation assay and DUOlink in situ proximity ligation were used to investigate the interaction between PKCβ1 and ERα.

Results: Endx more potently inhibited PKCβ1 kinase activity compared to Tam ( IC50 350 nM vs 47.8 μM ) with KDs for PKCβ1 binding as follows: Endx (100 nM ), Tam ( 2 μM ), 4-HT ( 2 μM ) and NDMT (> 7 μM ). None of the SERMs exhibited PKCβ2 binding. In the MCF7/AC1 and MCF7/AC1 L-resistant cells, PKCβ1 knockdown resulted in ERα degradation and potently inhibited cell proliferation. These results were confirmed in T47D and MDA-MB-361 cells. Notably, PKCβ1 knockdown in MCF7/AC1 cells resulted in significantly greater E2 induced proliferation comparing siRNA knockdown vs. control. To further explore these effects, we evaluated the effects of PKCβ1 overexpression in MCF7 cells and demonstrated that PKCβ1 overexpression reduced cell proliferation and colony formation compared to parental MCF-7 cells without affecting ERα protein stability. Coimmunoprecipitation assays in transient transfected MCF-7 cells with exogenous PKCβ1 as well as PKCβ1 expressing MDA-MB-231 cells transiently or stably transfected with ERα demonstrated PKCβ1 and ERα interaction, with confirmation by Duolink assay that this interaction occurs in the cytoplasm.

Conclusions: Our findings demonstrated that endoxifen binds and inhibits PKCβ1 at relevant concentrations achieved in the endoxifen clinical trial studies. PKCβ1 interacts with cytoplasmic ERα and PKCβ1 knockdown inhibits cell proliferation and enhances ERα turnover. However, in PKCβ1 overexpressing cells, PKCβ1 may exhibit tumor suppressive effects. These data suggest a complex interaction between PKCβ1 and ERα and that endoxifen's effects on PKCβ1 may alter drug response of endocrine therapy. Further studies are ongoing to characterize the role of PKCβ1 and its role in ER biology and response to endoxifen.

Citation Format: Guo C, Kuffel MJ, Kudgus RA, Huang Z, Bode AM, Cheng J, Suman VJ, Reid JM, Bruinsma ES, Subramaniam M, Ames MM, Hawse JR, Goetz MP. Identification and characterization of a novel endoxifen substrate, PKCβ1, and its interaction with the estrogen receptor [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P1-08-03.

Abstract PD2-03: Final results of a first-in-human phase I study of the tamoxifen (TAM) metabolite, Z-Endoxifen hydrochloride (Z-Endx) in women with aromatase inhibitor (AI) refractory metastatic breast cancer (MBC) (NCT01327781)

Background: AI's are more effective than TAM in ER+ breast cancer. In AI refractory MBC, the response rate to TAM is 0% (Osborne 2011). Z-Endx is an active metabolite of TAM and among TAM treated women in the adjuvant and metastatic settings, reduced CYP2D6 metabolism and low Endx concentrations (Css <20 nM) have been associated with increased likelihood of disease recurrence. Preclinical studies have demonstrated greater Z-Endx exposure and anti-tumor activity with oral Z-Endx compared to equivalent doses of oral TAM (Reid 2014)

Methods: We conducted a phase I trial to determine the maximum-tolerated dose (MTD) and evaluate the toxicities, clinical activity, and pharmacokinetics (PK) of Z-Endx in patients (pts) with ER+, AI refractory MBC. Unlimited prior endocrine regimens were allowed. An accelerated titration schedule was applied (2 pts/dose level) until moderate toxicity or DLT, followed by a 3+3 design and then to expansion cohorts (40, 80, and 100 mg/day). Z-Endx was administered orally once daily (28 day cycle). Eye exams were performed at baseline, and end of cycles 2 and 6. PK was performed during cycle 1 and prior to subsequent cycles. For pts in the expansion cohorts, tumor biopsies were obtained at baseline for DNA sequencing (Foundation Medicine). Plasma cholesterol levels were obtained at baseline and after 1 cycle.

Results: From March 2011 to Dec 2014, 41 pts (38 evaluable), median age 60, received Z-Endx once daily encompassing 7 dose levels (20-160 mg/daily). The median number of prior hormonal regimens was 2 and 3 for the dose escalation and expansion cohorts, respectively. Dose escalation was stopped at 160 mg/day given MTD not reached and attainment of mean Endx Css of 3.6 uM. Cycle 1 DLT (PE) was observed in one patient (60 mg). No eye toxicity was observed. PK demonstrated mean Endx Css of > 1 uM at all dose levels ≥ 40 mg/day. Antitumor activity was observed at multiple dose levels including 3 confirmed partial responses and an additional 7 with stable disease for ≥6 cycles. Of these 10 pts, 9 had prior progression on both AI and fulvestrant and 3 additionally on TAM. After 1 cycle, total and LDL cholesterol decreased > 20 points in 54% and 40% of pts, respectively. Tumor sequencing in the expansion cohorts (n=14) did not identify ESR1 mutations; however, ESR1 amplification was identified in 1 pt with prolonged stable disease (>200 days). Of 6 pts with rapid progression (≤2 cycles), 4/6 had either CCND1 amplification (n=1) or at least one of the following activating mutations: ERBB2 L755S (n=1), AKT1 E17K (n=1), MTOR E1799K (n=1).

Conclusions: The direct administration of Z-END provides substantial drug exposure, acceptable toxicity, and "proof of principle" antitumor activity in endocrine resistant MBC. While the MTD was not determined, the goal of achieving Endx Css concentrations of > 1 uM was achieved. Tumor sequencing identified pts with predicted and confirmed endocrine resistance. A randomized phase II comparing endoxifen (80 mg/day) with TAM in AI refractory MBC was recently activated (NCT02311933). Supported in part by CA 133049, CA186686, CA15083, CA116201, and CA15083.

Citation Format: Goetz MP, Suman VJ, Reid JM, Northfelt DW, Mahr MA, Dockter T, Kuffel M, Buhrow SA, Safgren SL, McGovern RM, Collins JM, Streicher H, Hawse JR, Haddad TC, Erlichman C, Ames MM, Ingle JN. Final results of a first-in-human phase I study of the tamoxifen (TAM) metabolite, Z-Endoxifen hydrochloride (Z-Endx) in women with aromatase inhibitor (AI) refractory metastatic breast cancer (MBC) (NCT01327781). [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr PD2-03.

Abstract PD10-08: Venlafaxine inhibits the CYP2D6 mediated metabolic activation of tamoxifen: Results of a prospective multicenter study: (NCT00667121)

Background: CYP2D6 is the rate limiting enzyme responsible for the metabolic activation of tamoxifen (tam) to endoxifen. Compared to CYP2D6 poor metabolizers (PM), tam-treated CYP2D6 extensive metabolizers (EM) have higher endoxifen concentrations, more vasomotor symptoms (Goetz, MP J Clin Oncol 2005), and are more likely to discontinue tam (Rae, JM 2009. Pharmacogenomics J). Additionally, higher endoxifen concentrations are associated with a stepwise increase in tam side-effects (Lorizio, W Breast Cancer Res Treat 2012). The data regarding CYP2D6 genotype and recurrence is mixed. Venlafaxine is a weak CYP2D6 inhibitor not known to alter tam pharmacokinetics (PK) and commonly recommended for tam induced hot flashes. We conducted a multicenter pharmacological study to determine whether venlafaxine altered the PK of tam and to determine the distribution of CYP2D6 genotypes in this population

Methods: Women taking tam for at least 4 weeks and for whom venlafaxine was recommended for the treatment of hot flashes were eligible. Blood samples were collected prior to and 8–16 weeks following initiation of venlafaxine for steady state tam and metabolites. Genotyping was performed for alleles associated with no (PM; *3, *4, *5,*6); reduced (intermediate, IM; *10, 17 and *41); and ultra-rapid (UM; *1×2) metabolism. Power calculations demonstrated that 17 patients with paired samples were required (two-sided alpha=0.05 t-test, 90% power) to detect a 25% change in endoxifen levels after at least 8 weeks of concurrent treatment.

Results: 30 women (median age 48.5) initiated venlafaxine. CYP2D6 genotypes were within Hardy Weinberg Equilibrium (HWE). CYP2D6 UM allele frequency (6.7%) was higher while CYP2D6 *4 (13.3%) was lower than expected compared to an unselected population (0.5 and 21% respectively; Sachse Am. J. Hum. Genet. 1997), resulting in the absence of CYP2D6 PM/PM. Mean (min/max) baseline endoxifen concentrations (8.73; 1.5–20.5 ng/ml) were correlated with CYP2D6 phenotype as follows: intermediate (EM/PM, PM/IM): 6.8 (1.5–11.2); extensive (EM/EM, EM/IM): 9.4 (1.5–20.5) and ultra-rapid (UM/EM: 11.0; 7.8–14) (r2 = 0.35 p = 0.05). In patients with paired samples (n = 20), venlafaxine resulted in a 23% decrease in endoxifen (−2.06 ng/ml; 95% CI −0.69 to −3.04; p = 0.004), but not tam, NDMT, or 4HT concentrations. Following initiation of venlafaxine, CYP2D6 genotype was no longer associated with endoxifen concentrations (r2 = 0.28 p = 0.23). For women with reduced CYP2D6 metabolism [EM/PM (n = 9) or PM/IM (n = 1)], venlafaxine lowered endoxifen concentrations (−2.98 ng/ml) to a level (5.41 ng/ml) reported to be associated with a higher risk of recurrence in adjuvant tam treated patients (Madlensky, L Clin Pharmacol Ther 2011).

Conclusions: In this study, women with tam-induced vasomotor symptoms requiring venlafaxine were comprised predominantly of CYP2D6 EM and UM metabolizers. Venlafaxine significantly decreased endoxifen concentrations. Although the optimal concentration of endoxifen is unknown, given prior data linking low endoxifen concentrations with recurrence, venlafaxine should be used with caution in tam treated patients. (Supported by R01CA133049)

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr PD10-08.

PD01-06: Endoxifen Exhibits Potent Anti-Tumor Activity and Regulates Different Genes Than Tamoxifen in an Aromatase Expressing MCF7 Model Resistant to Letrozole

Background: First in human studies of Z-endoxifen hydrochloride (E), the active metabolite of tamoxifen (T), are underway in metastatic breast cancer (BC). Previous data have demonstrated the superiority of aromatase inhibitors (AI's) over T in estrogen receptor (ER) + BC. Using an in vivo aromatase expressing model (MCF7/AC1), we compared the antitumor activity of E with T and Letrozole (L), as well as the antitumor activity and global gene expression changes of E with T in an L-resistant model.

Methods: MCF7/AC1 tumors were stimulated with androstenedione. Once tumor size reached 300 mm3, mice (30/group) were randomly assigned to one of five treatment groups: control (daily, po), T (500 μg/day, sc), endoxifen 25 mg/kg/day p.o.(LDE) endoxifen 75 mg/kg/day p.o. (HDE) or letrozole, 10 μg/day s.c for 4 weeks. Tumors were harvested from control, T, and E groups while the L group continued treatment until the development of resistance defined as an increase in tumor volume of at least 300% from day 1. Mice with L-resistant tumors were randomly assigned to T (n=4) or E (n=5) for 4 weeks and then sacrificed. Gene expression in L-resistant tumors was quantified using Affymetrix U133+2 and changes in gene expression profiles [comparing T and E with L-resistant (n=3)] were analyzed. Genes identified as significantly different were confirmed by real-time RT-PCR assays.

Results: At the 4 week time point, both doses of E and L resulted in greater anti-tumor activity than control (Wilcoxon rank sum test: all p < 0.0001); however, tumor burden did not differ between T and control (p=0.095). HDE resulted in significantly less tumor burden than T (p=0.002) but was similar to L. In mice that continued on L, resistance developed at 24 weeks in 9/25 mice. These mice were randomly assigned to either T (n=4) or E (n=5) for 4 weeks. Tumor volume (expressed as a% of its size prior to randomization) was significantly different comparing E (73.3%; range: 69.3 to 80.75%) versus T (148.39%; range: 114.07 to 165.99%) (Wilcoxon rank sum test p=0.016). Compared to control, microarray studies identified 1518 unique probe sets regulated by E (p<0.001) compared to 441 for T including estrogen-regulated genes such as progesterone receptor (PGR) and amphiregulin (AREG) that were significantly down-regulated in the E group [PGR (−6.2 fold, p=0.000008) and AREG (−3.2 fold, p=.0006) but unchanged or up-regulated in the T group (PGR unchanged and AREG +9.2 fold p=0.00002). These findings were confirmed by RT-PCR.

Conclusions: Using the MCF7/AC1 model previously used to show the superiority of AI's over T, HDE demonstrated similar antitumor activity to L and was superior to T. In cells resistant to L, E was superior to T and gene expression changes demonstrate that E down-regulates while T activates estrogen regulated genes. These findings support the ongoing development of E for the treatment of ER+ BC.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr PD01-06.

Tamoxifen, HER2, and Endoxifen: The Role of CYP2D6 as a Predictor of Tamoxifen Resistance in ER+/HER2+ Breast Cancer

Background: Endoxifen, the active metabolite of tamoxifen (Tam), is currently being developed as a drug for the treatment of estrogen receptor (ER) + breast cancer (BC). HER2 expression in ER+ BC is associated with Tam resistance, and in vivo Tam administration to mice bearing ER+/HER2+ xenografts stimulates BC growth (Shou, JNCI 2004). In humans, endoxifen is the most important Tam metabolite responsible for inhibiting estrogen induced BC growth (Wu, Cancer Research 2009). CYP2D6 metabolism affects the concentrations (conc) of endoxifen (Stearns, JNCI, 2003) and associated with worse disease free survival (DFS) in Tam treated BC (Goetz, JCO, 2005). We sought to determine the activity of Tam and its metabolites in ER+/HER2+ BC cell lines and to evaluate the role of CYP2D6 metabolism in Tam-treated patients (pts) with ER+/HER2+ BC. Additionally, we sought to determine endoxifen conc in mice administered oral Tam.Methods: MCF7 (parental and HER2-expressing) and BT474 (ER+/HER2+) cells were used to compare the activity of Tam, 4HT, and endoxifen on estrogen- stimulated growth. Oral tam PK were characterized in mice treated with standard dose of Tam (4 mg/kg; 100 μg). Clinical data were obtained via a retrospective analysis of Tam-treated pts with ER+/ HER2+ BC randomized to receive 5 years of Tam (NCCTG 89-30-52). CYP2D6 metabolism (extensive or decreased) was based on CYP2D6 genotype (*3, 4, 6, 10, 17, 41) and co-administration of a CYP2D6 inhibitor (yes/no). HER2 was determined by immunohistochemistry (IHC) or FISH (tumors 0, 1, or 2+ by IHC). The association between CYP2D6 and DFS was assessed using the log-rank test and proportional hazards modeling.Results: Compared to Tam, endoxifen potently inhibited the growth of estrogen- stimulated BT474 cells. In MCF7 cells, expression of HER2 shifted the conc of endoxifen required for 50% inhibition of growth (IC50) from 54 nM (parental) to 131 nM (HER2 expressing). Using the range of conc of Tam and its metabolites observed in humans (Tam, 300-500 nM; 4HT, 5-10 nM; and endoxifen, 10-200 nM), only endoxifen potently inhibited estrogen- stimulated growth of MCF7HER2+ cells and only at conc achievable in CYP2D6 extensive metabolizers (>50nM). In mice, conc of 4HT and endoxifen were below 15 nM following an oral dose of 4 mg/kg. In NCCTG 89-30-52, both CYP2D6 phenotype and HER2 status was determined in 201/256 randomized pts. HER2 was expressed in 23/215 (11%) but not associated with DFS overall (p=0.62). In the HER2+ subset, pts with decreased CYP2D6 metabolism (n=10) had significantly shorter DFS compared to extensive metabolizers (n=9) (HR 9.5, p=0.03; 95% CI 1.16-76.9).Conclusions: Our in vitro and clinical data provide a simple pharmacological model for understanding HER2 resistance in Tam-treated breast cancer. Mice, which lack the CYP2D6 enzyme, may not be an appropriate model for understanding tam resistance given low conc of both 4HT and endoxifen. Given that the in vitro conc of endoxifen needed to inhibit the growth of ER+/HER-2+ BC are achievable in only a subset of humans (CYP2D6 extensive metabolizers), the primary administration of endoxifen could overcome de novo Tam resistance in ER+/HER2+ BC.

Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 2006.

Preclinical pharmacology and toxicology of intravenous MV-NIS, an oncolytic measles virus administered with or without cyclophosphamide

MV-NIS is an oncolytic measles virus encoding the human thyroidal sodium iodide symporter (NIS). Here, we report the results of preclinical pharmacology and toxicology studies conducted in support of our clinical protocol "Phase I Trial of Systemic Administration of Edmonston Strain of Measles Virus, Genetically Engineered to Express NIS, with or without Cyclophosphamide, in Patients with Recurrent or Refractory Multiple Myeloma." Dose-response studies in the KAS-6/1 myeloma xenograft model demonstrated a minimum effective dose of 4 x 10(6) TCID50 (tissue culture infectious dose 50)/kg. Toxicity studies in measles-naive squirrel monkeys and measles-susceptible transgenic mice were negative at intravenous doses up to 10(8) and 4 x 10(8) TCID50/kg, respectively. Abundant viral mRNA, maximal on day 8, was detected in cheek swabs of squirrel monkeys, more so after pretreatment with cyclophosphamide. On the basis of these data, the safe starting dose of MV-NIS for our clinical protocol was set at 1-2 x 10(4) TCID50/kg (10(6) TCID50 per patient).

Metabolic activation of dacarbazine by human cytochromes P450: the role of CYP1A1, CYP1A2, and CYP2E1

Dacarbazine (DTIC), a widely used anticancer agent, is inactive until metabolized in the liver by cytochromes P450 to form the reactive N-demethylated species 5-[3-hydroxymethyl-3-methyl-triazen-1-yl]-imidazole-4-carboxamide (HMMTIC) and 5-[3-methyl-triazen-1-yl]-imidazole-4-carboxamide (MTIC). The modest activity of DTIC in the treatment of cancer patients has been attributed in part to lower activity of cytochromes P450 (P450) in humans when compared with rodents. Importantly, the particular P450 isoforms involved in the activation pathway have not been reported. We now report that the DTIC N-demethylation involved in MTIC formation by human liver microsomes is catalyzed by CYP1A1, CYP1A2, and CYP2E1. The most potent inhibitors of DTIC N-demethylation were alpha-naphthoflavone (CYP1A1 and CYP1A2), quercetin (CYP1A2), chlorzoxazone (CYP1A2 and CYP2E1), and di-sulfiram (CYP2E1). Antihuman CYP1A2 antiserum also inhibited DTIC N-demethylation. DTIC N-demethylation in a panel of 10 human liver microsome preparations was correlated with the catalytic activities for CYP1A2 (ethoxyresorufin O-deethylation and caffeine N3-demethylation) in the absence of alpha-naphthoflavone and with the catalytic activities for CYP2E1 (chlorzoxazone 6-hydroxylations) in the presence of alpha-naphthoflavone. DTIC metabolism was catalyzed by recombinant human CYP1A1, CYP1A2, and CYP2E1. The Km (Vmax) values for metabolism of DTIC by recombinant human CYP1A1 and CYP1A2 were 595 microM (0.684 nmol/min/mg protein) and 659 microM (1.74 nmol/min/mg protein), respectively. The CYP2E1 Km value exceeded 2.8 mM. Thus, we conclude that (a) CYP1A2 is the predominant P450 that catalyzes DTIC hepatic metabolism; (b) CYP2E1 contributes to hepatic DTIC metabolism at higher substrate concentrations; and (c) CYP1A1 catalyzes extrahepatic metabolism of DTIC.

Indolocarbazole poisons of human topoisomerase I: regioisomeric analogues of ED-110

All four "symmetrical" regioisomers of ED-110, an indolocarbazole derivative having potent activity against human topoisomerase I (Topo I) were synthesized. The isomer containing hydroxyl groups in the 3- and 9-positions was approximately ten-fold more active against Topo I, and 5- to 35-fold more active against human solid tumor cell lines in vitro, relative to ED-110.

Phase I and pharmacological trial of fazarabine (Ara-AC) with granulocyte colony-stimulating factor

Fazarabine (1-beta-D-arabinofuranosyl-5-aza-cytosine, or Ara-AC) is a nucleoside analogue that consists of the arabinoside ring of 1-beta-D-arabinofuranosylcytosine and the pyrimidine base of 5-azacytidine. In Phase I and Phase II trials, neutropenia was dose limiting, with minimal nonhematological toxicity. The in vitro cytotoxic concentrations of Ara-AC could not be achieved in these studies; neutropenia precluded dose escalation. The objectives of this study were: to determine either the maximum tolerated dose of Ara-AC or to safely achieve target plasma levels of 2-5 microgram/ml when Ara-AC was administered as a 24-h infusion with granulocyte colony-stimulating factor (G-CSF) to patients with advanced refractory malignancies; to characterize the pharmacokinetic behavior of Ara-AC with G-CSF; and to define the relationship of Ara-AC pharmacokinetics to toxicity. Twenty-four patients received 67 courses of Ara-AC at doses of 54-112 mg/m2/h. Dose-limiting toxicity was approached but not reached. Grade 3 or 4 neutropenia and nausea were the principle side effects. Steady-state plasma concentrations exceeded the minimum target concentration of 2 microgram/ml in all patients who received >/=78 mg/m2/h for 24 h. The maximum target concentration was approached during administration of 112 mg/m2/h for 24 h. The mean steady-state clearance was 475 +/- 103 ml/min/m2 and did not change with dose. One partial response was seen. One patient received 16 courses and another received 7 courses of therapy before progression. Ara-AC can be safely administered in doses that result in plasma concentrations of 2-5 microgram/ml, if it is given with G-CSF. Phase II trials of Ara-AC in selected malignancies are planned.

Comparative resistance of idarubicin, doxorubicin and their C-13 alcohol metabolites in human MDR1 transfected NIH-3T3 cells

The anthracycline analog idarubicin (ID) is useful in the treatment of leukemias, and is of further interest because of the unique activity of its major circulating metabolite idarubicinol (IDOL). In vitro studies have shown that ID retains activity against tumor cells made resistant by prolonged exposure to substrates of the p-glycoprotein energy-dependent efflux pump. To selectively investigate multidrug resistance to ID in tumor cells, ID, IDOL, doxorubicin (DX) and doxorubicinol (DXOL) were evaluated for growth inhibitory activity when incubated with NIH-MDR1-G185 (MDR) cells or with the parent NIH-3T3 (3T3) cells. The MDR cells are transfected with the human multidrug gene mdr1, and express a functional p-glycoprotein. ID growth inhibitory activity was much less affected by p-glycoprotein-mediated efflux than was DX. ID IC50 values were only 1.8-fold greater in the MDR cell line than in the parental 3T3 cell line, while the IC50 value for DX was 12.3-fold greater in the transfected cell line. Verapamil (VRP) fully restored drug sensitivity of the MDR cell line to ID and DX. In studies with the alcohol metabolites, IDOL and DXOL IC50 values were 7.8- and 18.9-fold greater, respectively, for the MDR cell line than for the parental cell line. Intracellular concentrations of DX and DXOL, but not ID and IDOL, were substantially increased in the MDR cells when VRP was present in the incubation mixtures. ID and IDOL retain substantial growth inhibitory activity in mdr1-transfected cells, and ID may be of value in clinical settings where multidrug resistance mediated by p-glycoprotein is a potential limitation of therapy.

Murine pharmacokinetics and metabolism of penclomedine [3,5-dichloro-2,4-dimethoxy-6-(trichloromethyl)pyridine, NSC 338720]

Penclomedine, a highly substituted pyridine derivative, has been selected by the National Cancer Institute for evaluation as a potential anticancer agent based on antitumor activity observed in murine tumor models following i.v., p.o., and i.p. administration. We have developed a reverse-phase high performance liquid chromatography assay for PEN, and subsequently investigated murine pharmacokinetics and metabolism. Following rapid i.v. injection of PEN (300 mg/m2) to mice, plasma elimination was best described by a 2-compartment open model with an elimination phase half-life, total body clearance, and steady-state distribution volume of 69 min, 114 ml/min/m2, and 4800 ml/m2, respectively. While PEN displayed good p.o. absorption, bioavailability of PEN after p.o. administration was approximately 2% of that observed following i.v. administration. Metabolism contributed substantially to drug clearance, and total metabolites were slowly eliminated from plasma. After i.v. and p.o. administration of radiolabeled PEN, less than 0.2% of the parent drug was excreted in the 48-h urine, and 25-30% of the total radioactivity was recovered in urine. NADPH-dependent oxidative and reductive metabolism was observed when penclomedine was incubated with mouse microsomal preparations. Microsomal reductive metabolism of PEN led to formation of a metabolite tentatively identified as a molecule formed by dimerization of the radical species produced by cleavage of chlorine from the trichloromethyl moiety of penclomedine.

Anthracyclines and their C-13 alcohol metabolites: growth inhibition and DNA damage following incubation with human tumor cells in culture

Anthracyclines are important antitumor agents used in the treatment of solid tumors, lymphomas, and acute lymphoblastic as well as myelocytic leukemias. The clinical utility of agents such as doxorubicin and daunorubicin and their well-characterized cardiotoxicity have prompted many efforts to develop analogs that retain the desired spectrum of activity but are less cardiotoxic. One such analog is idarubicin (4-demethoxydaunorubicin), which is currently under study in the treatment of adult and pediatric leukemias. The major circulating metabolite of idarubicin is the alcohol product of ketoreductase biotransformation, idarubicinol. Following the administration of idarubicin to adult or pediatric patients, systemic exposure to idarubicinol is greater than that to idarubicin. Moreover, we have also documented the presence of idarubicinol in the cerebrospinal fluid of pediatric patients who have received idarubicin. Idarubicinol has been reported to have greater cytotoxic activity than other anthracycline alcohol metabolites, which are regarded as much less active products of metabolism. We therefore evaluated the growth-inhibitory and DNA-damaging activities of idarubicin, daunorubicin, doxorubicin, epirubicin, and their alcohol metabolites against three relevant (CCRF-CEM lymphoblastic leukemia, K562 myelogenous leukemia, and U87-MG glioblastoma) human tumor cell lines. We found that whereas idarubicin was 2-5 times more potent than the other three anthracycline analogs against these tumor cell lines, idarubicinol was 16-122 times more active than the other alcohol metabolites against the same three cell lines. In addition, idarubicinol and the parent drug idarubicin were equipotent, unlike the other anthracycline alcohol metabolites, which were much less cytotoxic than the corresponding parent drugs. We also assessed the ability of the four parent drugs and their alcohol metabolites to induce DNA single-strand breaks. Idarubicin was more potent than the other three anthracycline analogs and idarubicinol was much more effective than the other alcohol metabolites in inducing DNA damage. These studies in human leukemia and human glioblastoma cell lines support the hypothesis that idarubicinol plays an important role in the antitumor activity of idarubicin and that the activities of idarubicin and idarubicinol are related to their ability to damage DNA.

Preclinical pharmacology of the anthrapyrazole analog oxantrazole (NSC-349174, piroxantrone)

Oxantrazole (now designated as piroxantrone) is an anthrapyrazole analog under evaluation as a potentially useful anthracycline-like antitumor agent. In preparation for phase I clinical trials, we characterized certain aspects of oxantrazole preclinical pharmacology, including plasma stability, murine pharmacokinetics, in vitro/in vivo metabolism, and DNA damage following incubation with human tumor cells in culture. Oxantrazole was relatively unstable in fresh mouse and dog plasma and particularly unstable in fresh human plasma (t 1/2 less than 5 min at 37 degrees C). Its decomposition in plasma was prevented by the addition of ascorbic acid, suggesting oxidative degradation. Following rapid i.v. administration of oxantrazole to mice, plasma elimination was best described by a two-compartment open model with an elimination-phase half-life, total body clearance, and steady-state volume of distribution of 330 min, 458 ml/min per m2, and 87.9 l/m2, respectively. The c x t value calculated following i.v. administration of 90 mg/m2 oxantrazole to mice was 177 micrograms-min/ml. This value was subsequently used in a pharmacologically guided dose-escalation scheme for the oxantrazole phase I clinical trial. Oxantrazole was converted to a polar conjugate, presumably a beta-glucuronide, by rat but not mouse hepatic microsomal preparations and in vivo by the mouse. Oxantrazole introduced protein-associated DNA strand breaks following incubation with a human rhabdomyosarcoma cell line. Repair of the damage was complete by 15 h. Clinical pharmacologic studies are currently under way in conjunction with the phase I clinical trial of oxantrazole.