Oral paclitaxel and encequidar in patients with breast cancer: a pharmacokinetic, safety, and antitumor activity study.

Background: Paclitaxel is widely used for the treatment of metastatic breast cancer (MBC). However, it has a low oral bioavailability due to gut extrusion caused by P-glycoprotein (P-gp). Oral paclitaxel (oPAC) may be more convenient, less resource-intensive, and more tolerable than its intravenous form. Encequidar (E) is a first-in-class, minimally absorbed, gut-specific oral P-gp inhibitor that facilitates the oral absorption of paclitaxel. Objectives: To investigate the pharmacokinetics (PK), overall response rate (ORR), and safety of weekly oral paclitaxel with encequidar (oPAC + E) in patients with advanced breast cancer. Design: This is a multicenter, single-arm, open-label study in six medical centers in Taiwan. Methods: Patients with advanced breast cancer were administered 205 mg/m2 oPAC and 12.9 mg E for 3 consecutive days weekly for up to 16 weeks. Plasma samples were collected at weeks 1 and 4. PK, ORR, and safety were evaluated. Results: In all, 28 patients were enrolled; 27 had MBC; 23 had prior chemotherapy; and 14 had ⩾2 lines of prior chemotherapy. PK were evaluable in 25 patients. Plasma paclitaxel area under the curve (AUC)(0-52 h) at week 1 (3419 ± 1475 ng h/ml) and week 4 (3224 ± 1150 ng h/ml) were equivalent. Best overall response in 28 evaluable patients was partial response (PR) in 11 (39.3%), 13 (46.4%) stable disease (SD), and 1 (3.6%) with progressive disease (PD). No patient achieved complete response (CR). The clinical benefit rate (CR + PR + SD) was 85.7%. Major adverse events among the 28 treated patients were grade 3 neutropenia (25%), grade 4 neutropenia (18%), with febrile neutropenia in 4%, and grade 3 diarrhea (4%). No treatment-related deaths occurred. Grade 2 peripheral neuropathy occurred in 1 (4%) patient and grade 3 peripheral neuropathy in 1 (4%) patient. Conclusions: oPAC + E produced a consistent therapeutic plasma paclitaxel exposure during treatment. There was a high rate of radiologically assessed clinical benefit, and a low rate of neurotoxicity which may provide advantages over IV paclitaxel. Registration: ClinicalTrials.gov Identifier: NCT03165955.

Contact Sensitization and Phototoxic and Photoallergic Potential of Tirbanibulin 1% Ointment in Healthy Volunteers.

Tirbanibulin 1% ointment is approved for the topical treatment of actinic keratosis, applied once daily for 5 days. Three phase 1 randomized, single-center, controlled, within-subject comparison studies were conducted to evaluate the sensitization (KX01-AK-006), phototoxic (KX01-AK-008), and photoallergic (KX01-AK-009) potential of tirbanibulin 1% ointment in healthy adults. In KX01-AK-006 and KX01-AK-009, subjects received repeated applications of tirbanibulin or vehicle for induction (followed by irradiation in KX01-AK-009) and an additional application for the challenge on naïve sites. In KX01-AK-008, subjects received single applications, followed by irradiation. Sensitization was defined as a reaction scoring 3 at naïve sites, recurring at rechallenge. Photoallergy was assessed based on the dermal response of erythema + edema at naïve sites. Phototoxicity was assessed based on the average dermal response score (days 3‒4). Adverse events were collected. In KX01-AK-006, none of the 229 subjects scored 3 at naïve sites. In KX01-AK-008, none of the 31 subjects developed edema, not meeting the criteria for phototoxicity. In KX01-AK-009, none of the 59 subjects showed reactions compatible with photoallergy. Mild-to-moderate contact irritations were reported. The evidence provided by these phase 1 studies showed that tirbanibulin 1% ointment lacks sensitization and phototoxic or photoallergic potential, and supports the safety of its topical application.

Open-Label, Randomized, Multicenter, Phase III Study Comparing Oral Paclitaxel Plus Encequidar Versus Intravenous Paclitaxel in Patients With Metastatic Breast Cancer.

PURPOSE: Intravenous paclitaxel (IVpac) is complicated by neuropathy and requires premedication to prevent hypersensitivity-type reactions. Paclitaxel is poorly absorbed orally; encequidar (E), a novel P-glycoprotein pump inhibitor, allows oral absorption. METHODS: A phase III open-label study comparing oral paclitaxel plus E (oPac + E) 205 mg/m2 paclitaxel plus 15 mg E methanesulfonate monohydrate 3 consecutive days per week versus IVpac 175 mg/m2 once every 3 weeks was performed. Women with metastatic breast cancer and adequate organ function, at least 1 year from last taxane, were randomly assigned 2:1 to oPac + E versus IVpac. The primary end point was confirmed radiographic response using RECIST 1.1, assessed by blinded independent central review. Secondary end points included progression-free survival (PFS) and overall survival (OS). RESULTS: Four hundred two patients from Latin America were enrolled (265 oPac + E, 137 IVpac); demographics and prior therapies were balanced. The confirmed response (intent-to-treat) was 36% for oPac + E versus 23% for IVpac (P = .01). The PFS was 8.4 versus 7.4 months, respectively (hazard ratio, 0.768; 95.5% CI, 0.584 to 1.01; P = .046), and the OS was 22.7 versus 16.5 months, respectively (hazard ratio, 0.794; 95.5% CI, 0.607 to 1.037; P = .08). Grade 3-4 adverse reactions were 55% with oPac + E and 53% with IVpac. oPac + E had lower incidence and severity of neuropathy (2% v 15% > grade 2) and alopecia (49% v 62% all grades) than IVpac but more nausea, vomiting, diarrhea, and neutropenic complications, particularly in patients with elevated liver enzymes. On-study deaths (8% oPac + E v 9% IVpac) were treatment-related in 3% and 0%, respectively. CONCLUSION: oPac + E increased the confirmed tumor response versus IVpac, with trends in PFS and OS. Neuropathy was less frequent and severe with oPac + E; neutropenic serious infections were increased. Elevated liver enzymes at baseline predispose oPac + E patients to early neutropenia and serious infections (funded by Athenex, Inc; ClinicalTrials.gov identifier: NCT02594371).

Topical Tirbanibulin, a Dual Src Kinase and Tubulin Polymerization Inhibitor, for the Treatment of Plaque-Type Psoriasis: Phase I Results.

Plaque-type psoriasis is a common skin disorder. Tirbanibulin (KX01) is a new Src kinase inhibitor with potent antiproliferative activity against keratinocytes and has been approved for treatment of actinic keratosis. This Phase I study investigates the safety and activity of KX01 ointment in patients with plaque-type psoriasis. We recruited 28 patients from two medical centers in Taiwan. This study was performed in four stages. Double-blind treatments were randomized in stages I (KX01 0.01% + placebo, two rounds of two-week treatment) and II (KX01 0.1% + placebo, four weeks) and open-labelled in stages III (KX01 1%, five days) and IV (KX01 1%, five days weekly for four weeks). The safety, tolerability, KX01 concentration, target area score, physician global assessment, and disease relapse were determined. Most treatment-emergent adverse events were mild-to-moderate application site reactions. Three (50.0%) subjects from the stage IV group showed ≥50% reduction in the target area score (TAS50), while two subjects (33.3%) showed a clinically meaningful improvement in the physician global assessment score. KX01 0.01%, 0.1%, and 1% were safe and well-tolerated. KX01 1% at four weeks showed a promising activity for the treatment of plaque-type psoriasis.

Oral paclitaxel with encequidar compared to intravenous paclitaxel in patients with advanced cancer: A randomised crossover pharmacokinetic study.

AIMS: Paclitaxel is a widely used anti-neoplastic agent but has low oral bioavailability due to gut extrusion by P-glycoprotein (P-gp). Oral paclitaxel could be more convenient, less resource intensive, and more tolerable than intravenous administration. Encequidar (HM30181A) is a novel, minimally absorbed gut-specific P-gp inhibitor. We tested whether administration of oral paclitaxel with encequidar (oPac+E) achieved comparable AUC to intravenous paclitaxel (IVP) 80 mg/m2 . METHODS: We conducted a multi-centre randomised crossover study with two treatment periods. Patients (pts) with advanced cancer received either oral paclitaxel 615 mg/m2 divided over 3 days and encequidar 15 mg orally 1 hour prior, followed by IVP 80 mg/m2 , or the reverse sequence. PK blood samples were taken up to Day 9 for oPac+E and Day 5 for IVP. RESULTS: Forty-two patients were enrolled; 35 completed both treatment periods. AUC0-∞ was 5033.5 ± 1401.1 ng.h/mL for oPac+E and 5595.9 ± 1264.1 ng.h/mL with IVP. The geometric mean ratio (GMR) for AUC was 89.50% (90% CI 83.89-95.50). Mean absolute bioavailability of oPac+E was 12% (CV% = 23%). PK parameters did not change meaningfully after 4 weeks administration of oPac+E in an extension study. G3 treatment-emergent adverse events occurred in seven (18%) pts with oPac+E and two (5%) with IVP. Seventy-five per cent of patients preferred oPac+E over IVP. CONCLUSIONS: GMR for AUC was within the predefined acceptable range of 80-125% for demonstrating equivalence. oPac+E is tolerable and there is no evidence of P-gp induction with repeat administration. With further study, oPac+E could be an alternative to IVP.

Phase 3 Trials of Tirbanibulin Ointment for Actinic Keratosis.

BACKGROUND: The tubulin polymerization and Src kinase signaling inhibitor tirbanibulin is being investigated as a topical treatment for actinic keratosis, a precursor of squamous-cell carcinoma. METHODS: In two identically designed double-blind trials, we randomly assigned, in a 1:1 ratio, adults with actinic keratoses on the face or scalp to receive either topical tirbanibulin or vehicle (placebo) ointment. The ointment was applied by the patients to a 25-cm2 contiguous area containing four to eight lesions once daily for 5 consecutive days. The primary outcome was the percentage of patients with a complete (100%) reduction in the number of lesions in the application area at day 57. The secondary outcome was the percentage of patients with a partial (≥75%) reduction in the number of lesions within the application area at day 57. The incidence of recurrence was evaluated at 1 year. Local reactions were scored with the use of 4-point scale (ranging from 0 [absent] to 3 [severe]). RESULTS: A total of 702 patients were enrolled in the two trials (351 patients per trial). Complete clearance in trial 1 occurred in 44% of the patients (77 of 175) in the tirbanibulin group and in 5% of those (8 of 176) in the vehicle group (difference, 40 percentage points; 95% confidence interval [CI], 32 to 47; P<0.001); in trial 2, the percentages were 54% (97 of 178 patients) and 13% (22 of 173), respectively (difference, 42 percentage points; 95% CI, 33 to 51; P<0.001). The percentages of patients with partial clearance were significantly higher in the tirbanibulin groups than in the vehicle groups. At 1 year, the estimated percentage of patients with recurrent lesions was 47% among patients who had had a complete response to tirbanibulin. The most common local reactions to tirbanibulin were erythema in 91% of the patients and flaking or scaling in 82%. Adverse events with tirbanibulin were application-site pain in 10% of the patients and pruritus in 9%, all of which resolved. CONCLUSIONS: In two identically designed trials, tirbanibulin 1% ointment applied once daily for 5 days was superior to vehicle for the treatment of actinic keratosis at 2 months but was associated with transient local reactions and recurrence of lesions at 1 year. Trials comparing tirbanibulin with conventional treatments and that have longer follow-up are needed to determine the effects of tirbanibulin therapy on actinic keratosis. (Funded by Athenex; ClinicalTrials.gov numbers, NCT03285477 and NCT03285490.).

Discovery of Novel Dual Mechanism of Action Src Signaling and Tubulin Polymerization Inhibitors (KX2-391 and KX2-361).

The discovery of potent, peptide site directed, tyrosine kinase inhibitors has remained an elusive goal. Herein we describe the discovery of two such clinical candidates that inhibit the tyrosine kinase Src. Compound 1 is a phase 3 clinical trial candidate that is likely to provide a first in class topical treatment for actinic keratosis (AK) with good efficacy and dramatically less toxicity compared to existing standard therapy. Compound 2 is a phase 1 clinical trial candidate that is likely to provide a first in class treatment of malignant glioblastoma and induces 30% long-term complete tumor remission in animal models. The discovery strategy for these compounds iteratively utilized molecular modeling, along with the synthesis and testing of increasingly elaborated proof of concept compounds, until the final clinical candidates were arrived at. This was followed with mechanism of action (MOA) studies that revealed tubulin polymerization inhibition as the second MOA.

Novel Application of the Two-Period Microtracer Approach to Determine Absolute Oral Bioavailability and Fraction Absorbed of Ertugliflozin.

Ertugliflozin, a sodium glucose cotransporter-2 inhibitor, is approved in the United States for treatment of type 2 diabetes mellitus. A novel two-period study design with 14 C microtracer dosing in each period was used to determine absolute oral bioavailability (F) and fraction absorbed (Fa ) of ertugliflozin. Eight healthy adult men received 100-µg i.v. 14 C-ertugliflozin (400 nCi) dose 1 h after a 15-mg oral unlabeled ertugliflozin dose (period 1), followed by 100 µg 14 C-ertugliflozin orally along with 15 mg oral unlabeled ertugliflozin (period 2). Unlabeled ertugliflozin plasma concentrations were determined using high-performance liquid-chromatography tandem mass spectrometry (HPLC-MS/MS). 14 C-ertugliflozin plasma concentrations were determined using HPLC-accelerator mass spectrometry (AMS) and 14 C urine concentrations were determined using AMS. F ((area under the curve (AUC)p.o. /14 C-AUCi.v. )*(14 C-Dosei.v. /Dosep.o. )) and Fa ((14 C_Total_Urinep.o. /14 C_Total_Urinei.v. )* (14 C-Dosei.v. /14 C-Dosep.o. )) were estimated. Estimates of F and Fa were 105% and 111%, respectively. Oral absorption of ertugliflozin was complete under fasted conditions and F was ∼100%. Ertugliflozin was well tolerated.

Vorapaxar, an oral PAR-1 receptor antagonist, does not affect the pharmacokinetics of rosiglitazone.

PURPOSE: To evaluate the potential effects of vorapaxar on the pharmacokinetics and safety of rosiglitazone. METHODS: This was an open-label, two-period, two-treatment, fixed-sequence study in 18 healthy subjects. On Day 1, Period 1, subjects received a single dose of rosiglitazone 8 mg. In Period 2, subjects received vorapaxar 40 mg on Day 1, vorapaxar 7.5 mg once-daily on Days 2-7, and a single dose of rosiglitazone 8 mg on Day 7. Rosiglitazone and N-desmethylrosiglitazone pharmacokinetics were assessed alone (Period 1) and after coadministration with vorapaxar (Period 2). Vorapaxar and its M20 metabolite pharmacokinetics were assessed on Day 7, Period 2. Safety and tolerability were assessed throughout the study. RESULTS: Coadministration of rosiglitazone with vorapaxar had no effect on rosiglitazone or N-desmethylrosiglitazone pharmacokinetics. The ratio of geometric means (GMR) and 90% confidence intervals (CI) of the coadministration versus monotherapy for Cmax (GMR 95; 90% CI 88, 103) and AUC0-24 h (GMR 103; 90% CI 98, 108) were within the 80-125% bioequivalence criteria. The metabolite-to-parent exposure ratio with and without vorapaxar was unaltered. Coadministration of vorapaxar with rosiglitazone was generally well tolerated. CONCLUSION: Coadministration of vorapaxar with rosiglitazone or drugs metabolized via CYP2C8 is unlikely to cause a significant pharmacokinetic interaction.

An evaluation of the systemic bioavailability of mometasone furoate (MF) after oral inhalation from a MF/formoterol fumarate metered-dose inhaler versus an MF dry-powder inhaler in healthy subjects.

PURPOSE: This randomized, open-label, multiple-dose, two-period, crossover study compared the systemic bioavailability of mometasone furoate (MF) administered from a metered-dose inhaler containing MF and formoterol fumarate (F) (MF/F-MDI) versus MF administered from a single-ingredient dry-powder inhaler (MF-DPI). METHODS: Healthy, non-smoking adults, 18-65 years with body mass index 18-29 kg/m(2) (N = 12) received MF 800 µg/F 20 µg via MF/F-MDI or MF 800 µg via MF-DPI twice daily for 5 days separated by a 7-day period. MF pharmacokinetics (AUC(0-12 hour) , Cmax , and Tmax ) were measured at Day 1 and 5 after each treatment. Safety and tolerability were assessed. RESULTS: Systemic exposure to MF based on AUC(0-12 hour) was ∼25% lower following MDI versus DPI administration. The Day 5 geometric mean ratio (MDI/DPI) estimates (90% confidence intervals [CI]) for AUC(0-12 hour) and Cmax were 0.747 (0.61, 0.91) and 0.606 (0.49, 0.75), respectively. The accumulation index (R) value for MF was higher following MDI (3.81-fold) versus DPI administration (2.34-fold) indicative of prolonged absorption. The most common adverse events were tremor, headache, and catheter site pain. CONCLUSIONS: Systemic exposure to MF was lower following multiple-dose MF/F-MDI administration versus MF-DPI administration. The magnitude of this difference is not considered to be clinically important. MF/F-MDI was safe and generally well tolerated.

Evaluation of the effects of a high-fat meal on the oral bioavailability of a single dose of preladenant in healthy subjects.

The aim of this study was to evaluate the effect of food on the oral bioavailability of preladenant, a novel adenosine A(2A) receptor antagonist. This open-label, randomized, single-dose, 2-way crossover study evaluated the effects of a high-fat, high-calorie meal on the pharmacokinetics of preladenant and its metabolite (SCH434748) following oral administration of a single 25-mg preladenant capsule to 24 healthy subjects. When administered with food, the time of maximum concentration (T(max)) of preladenant was prolonged compared with administration in the fasting state. Whereas T(max) was increased from 0.9 hours to 2.6 hours and maximum concentration (C(max)) was decreased (from 212 ng/mL to 128 ng/mL), the extent of absorption (area under the plasma concentration-time curve from time 0 to time of final quantifiable sample, or AUC([tf])) was unaffected by the meal. Similarly, SCH434748 T(max) was prolonged in the fed state, and C(max) decreased from 43.7 ng/mL to 28.6 ng/mL. The assessment of AUC([tf]) and area under the plasma concentration-time curve from time 0 to infinity [AUC([I])] together suggests that the AUC for the metabolite remained unchanged. No serious, significant, or unexpected adverse events occurred. In summary, food delays absorption and reduces peak exposure (C(max)) but does not alter the extent of preladenant exposure (AUC). These small changes are unlikely to be of clinical importance. A single 25-mg dose of preladenant is safe and well tolerated in healthy subjects under both fed and fasting conditions.

A phase I multicenter study of continuous oral administration of lonafarnib (SCH 66336) and intravenous gemcitabine in patients with advanced cancer.

We conducted a phase I study to assess safety, pharmacokinetics, pharmacodynamics, and activity of lonafarnib plus gemcitabine. Subjects received oral lonafarnib twice daily and gemcitabine on days 1, 8, and 15 every 28 days; multiple dose levels were explored. Lonafarnib had no apparent effect on gemcitabine PK. Mean lonafarnib half-life ranged from 4 to 7 hr; median T(max) values ranged from 4 to 8 hr. Two patients had partial response; seven patients had stable disease at least 6 months. Oral lonafarnib at 150 mg a.m./100 mg p.m. plus gemcitabine at 1,000 mg/m(2) is the maximum tolerated dose with acceptable safety and tolerability.

Phase I and pharmacokinetic study of lonafarnib, SCH 66336, using a 2-week on, 2-week off schedule in patients with advanced solid tumors.

PURPOSE: This phase I study was performed to determine the safety profile, maximum tolerated dose (MTD) and biological activity of lonafarnib (SCH 66336). Single-dose and multi-dose pharmacokinetics were conducted. METHODS: Twenty-one patients with advanced solid tumors were enrolled. Each patient received single-dose administration on day 1, cycle 1 then switched to a twice daily (BID) dosing regimen on days 2-14 of a 28-day cycle; subsequent cycles continued BID dosing on days 1-14. Dose-limiting toxicity (DLT) was assessed during the cycle one; toxicity evaluation was closely monitored throughout the treatment. Radiographic scans were completed to assess tumor response. Blood and urine pharmacokinetics were evaluated on days 1 and 14 in cycle 1. SCH 66336- induced farnesylation inhibition was assessed via conversion of prelamin A to lamin in buccal mucosa. RESULTS: DLT and most common adverse events were diarrhea, fatigue, nausea and anorexia. No grade 3 or 4 hematological toxicities were observed. Nineteen of 21 patients were evaluable for response; short-term stable disease was observed in 5 patients. SCH 66336 systemic exposure increased with dose; however, drug accumulation was higher than projected. Renal excretion of parent drug was negligible. Farnesyl transferase inhibition was detected at the 200 and 300 mg BID doses. CONCLUSION: The MTD and recommended phase II dose is 200 mg BID on days 1-14 of a 28-day dosing regimen. The plasma concentration profile suggests the pharmacokinetics of SCH 66336 is dose and time dependent. Farnesyl transferase target inhibition was observed at doses of lonafarnib recommended for further study.

Effects of multiple-dose pegylated interferon alfa-2b on the activity of drug-metabolizing enzymes in persons with chronic hepatitis C.

PURPOSE: To examine the effect of pegylated interferon (PEG-IFN) alfa-2b on the activity of major drug-metabolizing enzymes. METHODS: This nonrandomized, open-label, multiple-dose study examined the effects of PEG-IFN alfa-2b on the activity of CYP450 1A2, 2 C8/9, 2D6, and 3A4 enzymes and N-acetyltransferase in subjects with chronic hepatitis C. Eligible subjects received PEG-IFN alfa-2b 1.5 µg/kg subcutaneously once weekly for 4 weeks (days 3, 10, 17, and 24). Oral probe substrates (dextromethorphan hydrobromide 45 mg, caffeine 200 mg, tolbutamide 500 mg, and dapsone 100 mg) were administered after a 10-h fast on days 1 and 25. Midazolam 4 mg was administered orally on days 2 and 26. Enzyme activity for each CYP450 isozyme and for N-acetyltransferase was estimated based on the ratios of the observed concentrations of the substrates and metabolites in plasma or urine samples. RESULTS: Twenty-six subjects enrolled in the study. Mean age was 44.3 years, mean weight was 78.9 kg, and mean body mass index was 26.3 kg/m(2). Multiple doses of PEG-IFN alfa-2b inhibited CYP1A2 activity to a limited extent (point estimate = 84.2%, 90% confidence interval [CI] 79-90), increased CYP2C8/9 activity to a limited extent (point estimate = 127.6%, 90% CI 115-142), increased CYP2D6 activity (point estimate = 167%, 90% CI 125-223), and had no effect on the activity of CYP3A4 or N-acetyltransferase. CONCLUSION: Weekly administration of PEG-IFN alfa-2b to subjects with chronic hepatitis C increased CYP2C8/9 and CYP2D6 activity in some individuals.

The effect of multiple doses of peginterferon alfa-2b on the steady-state pharmacokinetics of methadone in patients with chronic hepatitis C undergoing methadone maintenance therapy.

This multicenter, open-label study evaluated the effects of multiple doses of peginterferon alfa-2b on the steadystate pharmacokinetics of methadone in 20 adults with hepatitis C virus infection who were enrolled in a methadone maintenance program. All subjects received peginterferon alfa-2b 1.5 mug/kg/wk for 4 weeks and maintained their normal methadone regimen. Serial blood samples were collected immediately before the first and after the fourth peginterferon alfa-2b dose (day 23). At day 23, exposure to the active methadone R-enantiomer increased by approximately 15% following administration of peginterferon alfa-2b, with 90% confidence intervals just outside the bioequivalence criteria (range, 80%-125%). Similar increases in exposure (C(max), AUC(0-24), and AUC(last)) were observed with S-methadone and total methadone. Peginterferon alfa-2b was well tolerated. Peginterferon alfa-2b is associated with minor increases in exposure to methadone in individuals with hepatitis C virus infection; however, these increases are unlikely to be clinically meaningful and are not associated with any safety concerns.

Temozolomide in patients with advanced cancer: phase I and pharmacokinetic study.

STUDY OBJECTIVE: To determine the maximum tolerated dose, dose-limiting toxicity, pharmacokinetics, and potential antitumor activity of temozolomide administered as a single dose every 28 days. DESIGN: Open label, phase I, dose-escalation trial. SETTING: University-affiliated cancer center. PATIENTS: Eleven patients aged 33-73 years with a documented solid tumor or lymphoma who failed therapy of proven efficacy for their disease or had a disease for which no conventional therapy was available. INTERVENTION: Temozolomide 500 mg/m2 was administered as a single oral dose every 28 days. Doses were escalated to 750 or 1000 mg/m2. No intrapatient dose escalation was allowed. At least two patients were enrolled at each dose level. Patients who did not have progressive disease and did not experience a dose-limiting toxicity, or experienced a dose-limiting toxicity but were eligible for dose reduction, were eligible to continue on the study. MEASUREMENTS AND MAIN RESULTS: Pharmacokinetic analysis was performed for temozolomide and its active metabolite, 5-(3-methyltriazeno)-imidazole-4-carboxamide (MTIC). Neutropenia and thrombocytopenia were dose limiting at 1000 mg/m2. Temozolomide was absorbed rapidly (mean time to maximum concentration 1.4 hrs) and eliminated, with average half-life and apparent oral systemic clearance values of 1.8 hours and 97 ml/minute/m2, respectively. Mean systemic exposure to MTIC was 3.7% of temozolomide. No objective responses were observed. The maximum tolerated dose of temozolomide was 750 mg/m2. CONCLUSION: Temozolomide 750 mg/m2 administered orally every 28 days was well tolerated. Alternate temozolomide dosing schedules such as continuous daily administration may enhance antitumor activity through sustained depletion of the DNA repair protein O6-alkylguanine DNA alkyltransferase.