Annals of Oncology Advance Access originally published online on September 15, 2006
Annals of Oncology 2006 17(11):1709-1717; doi:10.1093/annonc/mdl282
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© 2006 European Society for Medical Oncology
phase I and pharmacokinetics |
A phase I safety, pharmacological and biological study of the farnesyl protein transferase inhibitor, tipifarnib and capecitabine in advanced solid tumors
1 Department of Pediatrics, Medical Oncology, and Pharmacology, University of Colorado Cancer Center, Aurora, CO, USA
2 Department of Medical Oncology, University of Virginia, Charlottesville, VA, USA
3 Johnson & Johnson Pharmaceutical Research and Development, Titusville, NJ, USA
4 Johnson & Johnson Pharmaceutical Research and Development, Beerse, Belgium
*Correspondence to: Dr L. Gore, University of Colorado Health Sciences Center at Fitzsimons, Pediatrics, Mail Stop 8302, PO Box 6511, Aurora, CO 80045, USA. Tel: 303-724-4011; Fax: 303-724-4015; E-mail: lia.gore{at}UCHSC.edu
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Abstract |
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Background: To evaluate the toxicity and pharmacological and biological properties of the farnesyl protein transferase (FPTase) inhibitor, tipifarnib (R115777, ZARNESTRATM) and capecitabine administered for 14 days every 3 weeks.
Patients and methods: Patients with advanced cancers received twice daily tipifarnib (100–500 mg) and capecitabine (1000–1125 mg/m2) for 14 days every 3 weeks. Pharmacokinetics of tipifarnib, capecitabine and 5-fluorouracil (5-FU) were determined. Peripheral blood mononuclear cells were analyzed for farnesylation of the HDJ2 chaperone protein and FPTase activity.
Results: Forty-one patients received 185 courses of treatment. Diarrhea and palmar–plantar erythrodysesthesia were dose limiting at 300 mg tipifarnib/1125 mg/m2 capecitabine b.i.d. When the capecitabine dose was fixed at 1000 mg/m2 b.i.d., neutropenia was dose limiting at 400 and 500 mg b.i.d. of tipifarnib. Capecitabine did not affect the pharmacology of tipifarnib at 100–300 mg b.i.d., although tipifarnib significantly increased the Cmax of 5-FU at 400 mg b.i.d. HDJ2 farnesylation and FPTase activity decreased between 200 and 400 mg b.i.d. doses of tipifarnib, without a dose–response relationship. Five patients demonstrated partial remissions and 11 patients maintained prolonged stable disease.
Conclusions: Tipifarnib and capecitabine are well tolerated at 300 mg/1000 mg/m2 b.i.d., respectively, resulting in biologically relevant plasma concentrations and antitumor activity. The recommended dose for further disease-focused studies is 300 mg b.i.d. tipifarnib and 1000 mg/m2 b.i.d. capecitabine, given for 14 days every 3 weeks.
Key words: capecitabine, farnesyltransferase inhibitor, pharmacokinetics, phase I, tipifarnib
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introduction |
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Farnesylation is the post-translational addition of a lipophilic farnesyl moiety to a protein, promoting localization to cellular membranes [1]. Many proteins require this localization for activity, including several that promote cell proliferation, such as Ras and Rho. Farnesylation is catalyzed by farnesyl protein transferase (FPTase). Tipifarnib (R115777, ZARNESTRATM) is a specific, non-peptidomimetic, orally bioavailable competitive inhibitor of FPTase with in vitro activity at low-nanomolar concentrations and broad activity in vitro and in human xenograft models regardless of wild-type or mutant ras expression [2, 3]. Clinical responses are observed regardless of ras status, indicating mechanisms other than ras inhibition.
The maximum tolerated dose (MTD) in patients with solid tumors on a 21- of 28-day schedule was 300 mg b.i.d. [4]. Common toxic effects are myelosuppression, nausea, vomiting, diarrhea, fatigue and rash; myelosuppression is usually dose limiting [4]. Phase I studies with tipifarnib indicated clinical activity in various solid tumors [4–6]; however, tipifarnib alone and in some combinations with other agents in larger phase II and III studies have been largely disappointing [7–12].
Capecitabine (XELODATM) is an orally administered fluoropyrimidine carbamate with broad-spectrum activity. It is converted to 5-fluorouracil (5-FU) via three enzymic steps, the last of which is carried out by thymidine phosphorylase, an enzyme frequently overexpressed in malignant compared with normal tissues, effectively concentrating the cytotoxic effects of 5-FU in malignant cells [13, 14]. Capecitabine is Food and Drug Administration approved for use in patients with metastatic breast cancer refractory to anthracyclines and paclitaxel (Taxol) [15].
The rationale for combining tipifarnib with capecitabine includes the known activity of capecitabine in a variety of solid tumors, nonoverlapping mechanisms of action between the two agents and the feasibility of an all-oral regimen [16]. Study objectives were to determine the MTD and dose-limiting toxicity (DLT) of tipifarnib combined with capecitabine, to investigate pharmacokinetic and biologic interactions between the two agents, to assess preliminary antitumor activity in patients with advanced cancer, to explore markers of biologic activity of tipifarnib in clinical specimens and to help establish the toxicity profile and pharmacologic basis of this combination for further disease-directed trials.
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patient selection
Patients were
18 years old with histologically confirmed advanced solid malignancies refractory to standard therapy or for whom no effective therapy existed. Additional criteria included the following: Eastern Cooperative Oncology Group (ECOG) performance status of two or less; adequate unassisted nutritional status; no radiotherapy, chemotherapy or investigational agents within 4 weeks (6 weeks for nitrosoureas or mitomycin C); adequate hematopoietic, hepatic and renal function; cumulative prior radiation therapy to 
30% of bone marrow space; no prior hematopoietic stem-cell rescue; no uncontrolled brain metastases; no other concurrent antitumor therapy; no visual disturbance requiring intervention beyond corrective lenses; no known hypersensitivity to imidazoles and/or 5-FU and no limitations to full study compliance. Informed consent was obtained according to federal and institutional guidelines.
drug administration
Tipifarnib was supplied by Johnson & Johnson Pharmaceutical Research and Development (Titusville, NJ) as 100-mg tablets. Capecitabine (XELODATM; Roche Laboratories, Nutley, NJ) is available as 150- and 500-mg film-coated tablets; doses were calculated in milligrams per square meter and rounded to the nearest tablet size.
The starting doses were tipifarnib 100 mg and capecitabine 1000 mg/m2, orally within 30 min of a meal, b.i.d., days 1–14 of a 21-day cycle. There was no intrapatient dose escalation. For pharmacokinetic analyses, single doses of tipifarnib and capecitabine were administered 3 days and 1 day, respectively, before initiating the combined therapy.
Patients were enrolled in standard cohorts of three until DLT was observed in the first course. DLT was defined as (i) non-hematologic toxicity grade 3 (excluding nausea and vomiting without optimal supportive care), (ii) grade 4 neutropenia lasting >5 days or associated with fever >100.5°C, (iii) grade 3 thrombocytopenia lasting >5 days, (iv) grade 4 thrombocytopenia or (v) treatment delay >21 days due to drug-related toxicity. The MTD was defined as the highest dose at which 1 of three to six patients experienced DLT in the first course. Twelve patients were to enroll at the MTD to confirm tolerability and establish the recommended phase II dose.
Interval toxic effects had to resolve to grade 1 or baseline before proceeding with treatment. Patients could receive successive courses of treatment until they withdrew consent, exhibited progressive disease or failed to resolve drug-related toxicity within 21 days of the start of the next course, or if study discontinuation was in their best interest.
The dose levels originally planned escalated tipifarnib from 100 to 300 mg in 100-mg increments, with a fixed twice a day dose of 1000 mg/m2 of capecitabine, followed by escalation of capecitabine to 1125 and 1250 mg/m2 b.i.d. Due to DLTs at the 300-mg tipifarnib/1125-mg/m2 b.i.d. capecitabine level, the escalation scheme was modified to fix the dose of capecitabine at 1000 mg/m2 b.i.d. and further escalate tipifarnib to 400 and 500 mg b.i.d. The definition of MTD was modified such that a single occurrence of grade 3 palmar–plantar erythrodysesthesia (PPE) would count independently of other toxic effects. For example, if at any dose level, a single occurrence of grade 3 PPE and a single occurrence of any other DLT were observed in the first three patients (i.e. one DLT attributed to capecitabine and one other DLT), then that dose level would be expanded. If further DLTs were observed, dose escalation would be halted. Otherwise, dose escalation proceeded.
For hematologic toxic effects, dose modifications for capecitabine were made according to the package insert with the following specifications. For one occurrence of grade 3 myelosuppression during a cycle, tipifarnib was maintained at 100% in subsequent cycles. If a second episode occurred, tipifarnib was reduced by 100 mg b.i.d. from the initial dose for subsequent cycles. If a patient experienced grade 4 myelosuppression during a cycle, the treatment was held until grade 1, and tipifarnib was reduced by 100 mg b.i.d. in subsequent cycles. Patients who experienced a second episode of grade 4 myelosuppression discontinued study treatment.
For non-hematologic toxic effects, dosing was interrupted for clinically significant grade 2 or any grade 3 or 4 event, until resolved to grade 1. Recurrent grade 2 non-hematologic toxicity warranted a 25% reduction of capecitabine and/or tipifarnib. At the first through third occurrence of grade 3 non-hematologic toxicity during a treatment cycle, both drugs were held until toxicity resolved to grade 1. For the first incident of grade 3 non-hematologic toxicity, tipifarnib was maintained at 100%. If a second or third episode of grade 3 non-hematologic toxicity was noted, tipifarnib was reduced by 100 mg b.i.d. from the starting dose in subsequent cycles. At the fourth occurrence of grade 3 non-hematologic toxicity, participation was discontinued. If a patient experienced one or two episodes of grade 4 non-hematologic toxicity during a cycle, treatment was held until grade 1, and capecitabine was reduced by 50% for each episode. Tipifarnib was reduced by 100 mg b.i.d. at the second occurrence of grade 4 non-hematologic toxicity in subsequent cycles. Patients who experienced a third episode of grade 4 non-hematologic toxicity despite dose reductions discontinued participation.
pre-treatment and follow-up clinical assessments
Within 4 weeks of starting, baseline radiologic studies and relevant tumor markers were obtained; within 2 weeks, a complete history and physical, ophthalmologic examination, electrocardiogram and pregnancy test were carried out.
Weekly evaluations included toxicity assessments graded according to the National Cancer Institute Common Toxicity Criteria, Version 2.0, a complete blood cell count with differential and routine serum electrolytes and chemistries. Before each subsequent course, an interval history, physical examination, coagulation studies and urinalysis were carried out. After every two cycles, tumor status was re-assessed. Tumor responses were confirmed after 28 days by World Heath Organization criteria for response. Ophthalmologic evaluations were carried out during every third cycle.
pharmacokinetic sampling
Samples were obtained after single doses of tipifarnib (run-in day 1, 48 h before capecitabine) and capecitabine (run-in day 3, 24 h before cycle 1/day 1), and on cycle 1/day 1. On run-in day 1, blood was drawn immediately before tipifarnib administration, then 1, 2, 3, 5, 8, 12, 24 and 48 h after dosing. On run-in day 3, blood was drawn immediately before capecitabine dosing, then at 30 min and 1, 2, 3, 4, 5, 8 and 12 h after dosing. On cycle 1/day 1, blood was drawn immediately before capecitabine and tipifarnib dosing, then at 1, 2, 3, 4, 5, 8 and 12 h (before the evening capecitabine dose). An additional sample was drawn on cycle 1/day 15.
pharmacokinetic assay and analysis
Chromatographic peaks of tipifarnib (retention time 
4.3 min) and the internal standard (R121550, retention time 6.3 min) were quantified using UV detection at 240 nm by previously described methods [6].
Capecitabine and 5-FU plasma concentrations were determined using a validated high performance liquid chromatography (HPLC)-UV method. Briefly, drugs were extracted from 200 µl plasma using mixed mode solid phase extraction after plasma protein precipitation with acetonitrile. The extracts were analyzed by HPLC with UV detection at 302 nm. Chromatography was carried out in a 10-cm x 4.6-mm C18 base deactivation silica (BDS)-Hypersil column. 5-FU and its 15N2-labeled internal standard were extracted, transformed into their pentafluorobenzyl derivatives and analyzed by gas chromatography–mass spectrometry with negative chemical ionization.
The pharmacokinetic parameters of tipifarnib, capecitabine and 5-FU were calculated by non-compartmental methods using WinNonLin software (Version 3.1; Pharsight Corporation, Cary, NC). Maximum plasma concentrations (Cmax) and times to maximum plasma concentration (tmax) were determined from the plasma concentration–time profiles. Areas under concentration–time curves (AUC) were calculated by linear trapezoidal summation. The terminal rate constant (
z) of tipifarnib was calculated by linear regression of the terminal portion of the concentration–time profile. The terminal half-life of tipifarnib was calculated as the ratio 0.693/z.
Effects of single-dose tipifarnib on the pharmacokinetics of capecitabine and its active metabolite, 5-FU, were evaluated using analysis of variance (ANOVA). A separate analysis was carried out for each dose. The point estimates and corresponding 90% confidence intervals (CIs) about the least-squares mean ratio of capecitabine Cmax and AUClast and 5-FU Cmax and AUClast (with and without tipifarnib) were calculated using ANOVA. Similarly, the effect of single-dose capecitabine on the pharmacokinetics of tipifarnib (Cmax and AUC12) was evaluated. Analyses were carried out on logarithmically transformed pharmacokinetic parameters using SAS software (Version 6.21; SAS Institute, Inc., Cary, NC).
biological studies
peripheral blood mononuclear cell lysate collection and preparation.
Blood was collected in histopaque tubes (Sigma, St Louis, MO) at baseline, days 1 and 15 of cycles 1 and 2, and when possible, at termination of treatment. The lymphocyte fraction was isolated, erythrocytes were lysed and samples were centrifuged at 700 g for 10 min, frozen and stored at –70°C. Total protein was obtained with radioimmunoprecipitation assay (RIPA) buffer plus a protease inhibitor cocktail (Sigma), and quantified using the Bradford method [17].
western blot for HDJ2 farnesylation status.
Changes in the farnesylation of the chaperone protein HDJ2 in peripheral blood mononuclear cell (PBMC) lysates after exposure to tipifarnib were assayed by standard immunoblotting. Approximately 10–20 µg of PBMC lysate protein from each time point was mixed 1 : 1 with 2x Laemmli buffer (Biorad, Hercules, CA), loaded on to a 10% Tris–glycine polyacrylamide gel and run for 1 h at 200 V. Samples were electroblotted to a polyvinyl difluoride (PVDF) membrane (Amersham, Piscataway, NJ), blocked with 5% nonfat dried milk in tris-buffered saline (TBS)–Tween buffer with constant agitation for 1 h and then exposed to HDJ2 antibody (Lab Vision, Fremont, CA) for 1.5 h. After washing, samples were exposed to anti-mouse horseradish peroxidase-conjugated antibody (Amersham) for 1 h, washed and exposed to film for 1 min.
ChemiImager 5500 software (Alpha Innotech, San Leandro, CA) was used to quantify band intensity. The effects of tipifarnib on HDJ2 farnesylation were assessed by analyzing the mobility gel shift, comparing 44 to 46 kDa. The percentage of unfarnesylated HDJ2 was determined for each time point by dividing the value of each 46-kDa band by the sum of the values of the 46-kDa and the corresponding 44-kDa bands.
scintillation proximity assay of farnesyltransferase inhibition.
Changes in the enzymic activity of farnesyltransferase (FT) in PBMC lysates before, during and after exposure to tipifarnib were measured by scintillation proximity assay (SPA; Amersham). Counts per minute for each sample were determined on a Beckman LS1801 (Beckman, Fullerton, CA).
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general
Forty-one patients received 185 courses of tipifarnib and capecitabine (Table 1). All were assessable for toxicity. Ninety-five percent of patients had an ECOG performance status of 0 or 1. All but one had received prior therapy and 24 had prior 5-FU and/or capecitabine. Table 2 illustrates patients and courses listed by dose level and the dose escalation scheme. Seventeen of 41 subjects (41%) received four or more cycles of therapy. Twenty-five (61%) required delay in re-treatment due to toxicity in the previous cycle.
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hematologic toxicity
Neutropenia predominated over thrombocytopenia (Table 3). For all patients, absolute neutrophil count nadir occurred between days 18 and 21 (range 8–28) and resolved by day 29 in 184 of 185 (99%) courses. The median duration of grade 3 or 4 neutropenia was 7 days (mean = 8 days, range 1–28 days). Grade 3 or 4 neutropenia occurred in 19 (10.2%) of the overall courses, all in heavily pre-treated patients. Neutropenia was dose limiting in one of 12 patients, and the two patients in the tipifarnib 400 and 500 mg b.i.d./capecitabine 1000 mg/m2 b.i.d. cohorts. All three patients experiencing hematologic DLT were heavily pre-treated with multiagent chemotherapy; two of the three also had
50 Gy of prior radiotherapy, and one, to a marrow-rich area (pelvis). Treatment delays due to neutropenia up to 22 days (range 7–22 days) were required in eight of 185 courses (4.3%), and occurred in the first two cycles of therapy for one of nine at the 300/1000 b.i.d. level, three of 12 at the 400/1000 b.i.d. level and two of two at the 500/1000 b.i.d. level. All treatment delays for neutropenia occurred in heavily pre-treated patients, but were not cumulative in any patient.
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Grade 3 or 4 thrombocytopenia occurred in six (3%) of 185 courses, lasting a mean of 5.5 days (range 1–8), but was dose limiting in only two patients (1%) of courses, who also experienced concomitant grade 4 neutropenia. Both these patients were treated at the 500/1000 b.i.d. dose, and were heavily pre-treated.
non-hematologic toxicity
The most common toxic effects were mild-to-moderate PPE (5% of courses, grade 1–2 severity), nausea/vomiting and diarrhea (Table 4). Grade 1–2 gastrointestinal toxic effects included nausea (17%), vomiting (11%), anorexia (7%) and diarrhea (9%). Over all cycles, grades 3 and 4 non-hematologic toxic effects consisted of five episodes of grade 3 fatigue (one at 100/1000 b.i.d., three at 400/1000 b.i.d. and one at 500/1000 b.i.d.), lasting for a median of 13 days (range 8–20 days). Two episodes of grade 3 vomiting were noted at 200/1000 b.i.d., each lasting 1 day; three episodes of grade 3 diarrhea, lasting 5 days at 300/1125 b.i.d. and 1 day and 15 days at 400/1000 b.i.d.; two episodes of grade 3 nausea at 200/1000 b.i.d., each lasting 1 day. Tipifarnib dose was reduced for non-hematologic toxicity in two of 12 patients at 400/1000 b.i.d. Capecitabine dose was reduced for non-hematologic toxicity in three of 12 patients at 400/1000 b.i.d.
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dose intensity/dose modifications
The median dose intensity of tipifarnib was
90% of planned. The median dose intensity of capecitabine was within 90% of planned for the cohorts at or below the MTD. Overall, 15 of 41 patients (37%) required dose reductions of capecitabine, and six of 41 patients (15%) required dose reductions of tipifarnib due to toxicity. Twenty-one of 41 subjects (51%) experienced at least one grade 3 or 4 adverse event during therapy, considered related to tipifarnib and/or capecitabine.
antitumor activity
Five patients, all of whom had previous therapy, had confirmed responses. A 48-year-old woman with metastatic colon cancer who had received 5-FU/leucovorin 17 months before study entry, treated at 200/1000 b.i.d., exhibited a decline in carcinoembryonic antigen from 1362 to 336 after six cycles of therapy, associated with a >50% reduction in abdominal soft-tissue disease. At 300/1000 b.i.d., a 75-year-old woman with breast cancer metastatic to chest wall, cervical lymph nodes and brain experienced a partial response (PR) in 10 target lesions (including brain) after two cycles, persisting for eight cycles. This patient had prior progression on paclitaxel, tamoxifen and vinorelbine. A PR was observed in a 45-year-old man with recurrent squamous cell carcinoma of the head and neck, who progressed on carboplatin, paclitaxel and 68 Gy of radiation therapy. This patient was treated at 300/1125 b.i.d., and experienced complete response (CR) of a 2-cm oropharyngeal mass and improvement of a lesion on the floor of the mouth after two cycles. The response lasted for six cycles. At 300/1125 b.i.d., a 53-year-old man with biliary squamous cell carcinoma and progression on cisplatin/5-FU experienced a >50% decrease in tumor size after two cycles, persisting for four additional cycles. Lastly, a 50-year-old man on 400/1000 b.i.d. with salivary duct carcinoma, who had prior surgery and 66 Gy of radiation, achieved a PR lasting for 10 cycles. Stable disease for more than four courses was seen in 11 patients. A 34-year-old woman with metastatic melanoma enrolled at dose level 1 maintained stable disease for 32 courses (24 months) and was discontinued due to concerns of cataract formation. A 2-[fluorine-18]fluoro-2-deoxy-D-glucose scan at that time showed an elevated standardised uptake value (SUV) in the tumor region, indicating persistent disease; however, she remained off additional therapy without further disease progression for 36 months. Seven of 16 patients (44%) with responses or prolonged disease stabilization had received previous 5-FU and/or capecitabine.
pharmacokinetics
Plasma samples for pharmacokinetic studies were obtained from 36 and 41 patients for tipifarnib with or without capecitabine, respectively, and from 36 and 40 patients for capecitabine and 5-FU with or without tipifarnib, respectively (Table 5).
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The maximum plasma concentrations of tipifarnib were observed
2–3 h after administration, regardless of whether it was administered alone or with capecitabine. On average, the Cmax and AUC12 of tipifarnib in combination with capecitabine (1000 or 1125 mg/m2 b.i.d.) were almost identical [1.0% (P = 0.87) and 2.4% higher (P = 0.60), respectively] to those following tipifarnib monotherapy.
Cmax of capecitabine was observed 1–3 h after dosing regardless of whether capecitabine was administered alone or with tipifarnib. Relative to capecitabine monotherapy, the Cmax values of capecitabine in combination with 100–300 mg tipifarnib b.i.d. were 2.9%–38.5% lower (P = 0.05–0.90), and the AUClast was 0.3%–27.8% lower (P = 0.08–0.98). In contrast, 400 mg tipifarnib b.i.d. resulted in a 67.1% increase in the Cmax (P = 0.08) and 34.8% increase in the AUClast (P = 0.07) of capecitabine. Maximum 5-FU plasma concentrations were observed 1–3 h after dosing. Similar to capecitabine, neither the Cmax nor the AUClast values were affected by 100–300 mg b.i.d. of tipifarnib, while 400 mg resulted in a large (67.8%, P = 0.02) increase in the mean Cmax of 5-FU but a smaller increase (15.8%, P = 0.11) in the mean AUClast. At this dose, the Cmax of 5-FU was 167 ± 76 ng/ml in the absence and 314 ± 211 ng/ml in the presence of tipifarnib. ANOVA of the 90% CIs for a single dose of tipifarnib on the pharmacokinetics of 5-FU is significant only for the Cmax at 400/1000 mg/m2 b.i.d. (90% CI = 122.8–229.1; P = 0.02). A suggestion of higher plasma concentrations of capecitabine and its active metabolite, 5-FU, was observed in subjects coadministered 500 mg tipifarnib b.i.d.; however, the sample size (n = 2) is too small to be meaningful.
biological studies
immunoblotting of HDJ2.
Figure 1A depicts a representative immunoblot from a patient in the 300/1125 b.i.d. cohort. This shows the 44-kDa (farnesylated) HDJ2 band at baseline and the appearance of a 46-kDa (unfarnesylated) HDJ2 band after 2 weeks of tipifarnib treatment. On cycle 2/day 1 (representing a 1-week washout of tipifarnib), treatment was delayed due to toxicity; the patient began cycle 2 day 1 dosing 1 week later. As demonstrated, the protein completely reverted to the processed form after 7–14 days off tipifarnib, but became proportionally less processed after the second 14 days of tipifarnib treatment. Inhibition of HDJ2 farnesylation was observed even at the lowest tipifarnib dose, and without clear evidence of a dose–response effect between the 200- and 400-mg b.i.d. doses in all 21 evaluable patient samples tested, as shown in Figure 1B.
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scintillation proximity assay.
Table 6 depicts the inhibitory activity of tipifarnib on FT, as a function of dose level. FT inhibition occurred at doses of tipifarnib between 200 and 500 mg b.i.d. with average values ranging from 30% to 47%, without an apparent dose–response relationship. Although the interpatient variability was high, the intrapatient variability was consistently
20%.
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discussion |
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This study was designed to explore the feasibility of the combination of tipifarnib, a non-peptidomimetic farnesyltransferase inhibitor (FTI), administered with capecitabine, an oral fluoropyrimidine carbamate, in patients with advanced cancer. Previous work has indicated feasibility of tipifarnib combined with 5-FU and leucovorin [16]. The rationale for this study was based on the antitumor activity of capecitabine in a variety of tumors, different mechanisms of action of the agents and the desirability of an all-oral regimen.
Similar to single-agent studies with tipifarnib, and with other FTIs, the DLT in this study was myelosuppression [4, 7, 8, 18–23]. All patients with hematologic DLT were heavily pre-treated with multiagent chemotherapy, and two of the three had previous radiation therapy >50 Gy. Patients were not stratified on the basis of prior therapy. Stratification might have provided data on the potential ability to deliver higher doses of tipifarnib, particularly to patients who were less heavily pre-treated. At 400/1000 b.i.d. dose, although only two patients experienced DLT, three of 12 patients had dosing delays due to neutropenia and five of 12 patients required dose reductions of either tipifarnib or capecitabine for non-hematologic toxicity. No patients at the 300/1000 b.i.d. dose experienced DLT; only one of nine patients required delay due to neutropenia, and no patients required dose reductions of either drug for non-hematologic toxicity. Thus, the recommended phase II dose on this schedule is tipifarnib 300 mg b.i.d. and capecitabine 1000 mg/m2 b.i.d.
Neutropenia was clearly related to prior myelotoxic therapy, but as seen with other FTIs, did not appear to be cumulative [8]. As in the single-agent tipifarnib study of Johnston and co-workers [18], significant thrombocytopenia in our study occurred in the context of grade 3 or 4 neutropenia. Hematologic toxicity in this and other studies has been attributed to a class effect of the FTIs [18].
Non-hematologic toxic effects were generally mild to moderate and similar to those of capecitabine, with anorexia and fatigue indicating a contribution from tipifarnib. Unlike previous studies with FTIs [6, 24, 25], tipifarnib neurotoxicity was not observed, due possibly to the interrupted dosing schedule [6, 18, 24, 25], as observed by Lara and co-workers [26].
Pharmacokinetic analyses indicate that tipifarnib Cmax and AUC12 were not affected by coadministration of capecitabine. Mean tipifarnib plasma concentration at 300/1000 b.i.d. is similar to that noted in previous clinical trials [8, 23, 27], and that required to inhibit farnesylation of Ras, other critical proteins and tumor growth [28]. At the 400/1000-mg b.i.d. dose, there was significant evidence of dose-dependent changes in the Cmax and AUClast of capecitabine and 5-FU with concomitant administration of tipifarnib. The wide intersubject variability in tipifarnib pharmacokinetics has been observed previously [12, 16, 22, 23]. No significant relationship between tipifarnib or capecitabine AUC and neutropenia was apparent using Emax models. There was a trend towards a linear relationship between dose and tipifarnib Cmax and AUC12.
Biomarker evaluation following tipifarnib treatment was carried out by analyzing HDJ2 chaperone protein and SPA. Although the numbers were small, consistent inhibition of HDJ2 farnesylation was observed at all tipifarnib levels over 100 mg b.i.d., without a dose–response relationship. The SPA results demonstrated a similar pattern and magnitude. In non-hematologic malignancies, such assays may not be useful as predictive biomarkers, but may characterize dose levels that are less likely to result in biological effects. In a phase I study of tipifarnib plus gemcitabine, comparable inhibition of farnesylation of HDJ2 also was noted across dose levels [29].
Five of 41 (12%) patients experienced confirmed responses (CR or PR) and an additional 11 patients had prolonged disease stabilization, for an overall disease control rate of 39%. All responding patients had previous treatment. Responses in this study were seen across all dose levels, with no suggestion of a dose–response relationship; however, all objective responses were noted at tipifarnib 200–300 mg b.i.d./capecitabine 1000–1125 mg b.i.d. doses, which are at or near the recommended dose defined. Interestingly, the response rate in this study is similar to a single-agent tipifarnib study of advanced breast cancer patients [18], although in that study, no confirmed responses were noted. Our study enrolled eight breast cancer patients, one heavily pre-treated patient exhibiting a confirmed PR for 24 weeks. Three other breast cancer patients in this study exhibited stable disease for 12, 15 and 30 weeks. In another phase I study of the FTI lonafarnib, combined with cisplatin and gemcitabine, two heavily pre-treated patients with highly refractory breast cancer (one with adenocarcinoma and one with inflammatory breast cancer, and both with prior fluoropyrimidine exposure) demonstrated a CR and a PR (72% tumor reduction) for 21 and 24 weeks, respectively [30]. Although the numbers are small, and the results confounded by the activity of single-agent capecitabine, these and other data indicate that the FTIs plus chemotherapy merit further study in breast cancer.
FTIs were originally developed to modulate the activity of ras gene products. Despite inhibition of farnesylation in clinical specimens in multiple clinical trials at tolerable doses or in primary tumor specimens, no correlation between ras mutation status and clinical activity has been identified [9–11, 18, 25]. Perhaps, tipifarnib is less effective inhibiting farnesylation of mutant K-RasB (the isoform most often seen in human cancer) than H- or N-Ras in pre-clinical testing, attributed, in part, to a higher affinity of K-RasB for FPTase [3]. Additionally, K-Ras, when exposed to FTIs, has the ability (shared with N-Ras and RhoB among others) to undergo geranylgeranylation [31], a distinct prenylation process, the enzyme for which is not inhibited by non-peptidomimetic FTIs. Similarly, the complex steps involved in isoprenylation and the inhibition of isoprenylation by the FTIs indicate another Ras-independent antitumor mechanism [32].
It is not clear that the true mechanism of action of the FTIs is related to effects on Ras or even on farnesylated proteins other than Ras. Ras may not be the key target of FPTase inhibitors [33–35]. A variety of proteins are involved in cellular proliferation and cell cycle regulation; RhoB, CENP-E, the lamins and others may be alternative FTI targets [32]. Until we have a better understanding of these targets, clinical development needs to rely on empirical promising leads discovered in early clinical trials such as this. Clinical studies coupled with further pre-clinical and molecular evaluation of the mechanism of antitumor activity of this combination will lead to a better understanding of the pharmacological interactions that lead to the clinical responses observed. Based on the results presented here, further studies of tipifarnib (300 mg b.i.d.) and capecitabine (1000 mg/m2 b.i.d.) are warranted.
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LG is the recipient of an NIH K12 Clinical Oncology Scholars Award (CA-86913) from the National Cancer Institute and the University of Colorado Cancer Center.
Received for publication May 17, 2006. Revision received July 3, 2006. Accepted for publication July 4, 2006.
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