|
Advertisement |
|
|
||
 |
|
|||||
|
|||||
|
 |
||||||
|
||||||
|
||||||
|
© 2000 American Society for Clinical Oncology Phase I and Pharmacologic Study of PN401 and Fluorouracil in Patients With Advanced Solid MalignanciesFrom the Institute for Drug Development, Cancer Therapy and Research Center and the University of Texas Health Science Center, San Antonio, TX; and Pro-Neuron, Inc, Gaithersburg, MD. Address reprint requests to Manuel Hidalgo, MD, Institute for Drug Development, Cancer Therapy and Research Center, 8122 Datapoint Dr, Suite 700, San Antonio, TX, 78229; email mhidalgo{at}saci.org
PURPOSE: To assess the feasibility of administering PN401, an oral uridine prodrug, as a rescue agent for the toxic effects of fluorouracil (5-FU), and to determine the maximum-tolerated dose of 5-FU when given with PN401, with an 8-hour treatment interval between these agents. PATIENTS AND METHODS: Patients with advanced solid malignancies were treated with escalating doses of 5-FU, given as a rapid intravenous infusion weekly for 3 consecutive weeks every 4 weeks. PN401 was administered orally 8 hours after 5-FU administration, to achieve sustained plasma uridine concentrations of at least 50 µmol/L. Initially, patients received 6 g of PN401 orally every 8 hours for eight doses (schedule 1). When dose-limiting toxicity (DLT) was consistently noted, patients then received 6 g of PN401 every 2 hours for three doses and every 6 hours thereafter for 15 doses (schedule 2). RESULTS: Twenty-three patients received 50 courses of 5-FU and PN401. Among patients on schedule 1, DLT (grade 4 neutropenia complicated by fever and diarrhea) occurred in those receiving 5-FU 1,250 mg/m2/wk. Among patients on schedule 2, 5-FU 1,250 mg/m2/wk was well tolerated, but grade 4, protracted (> 5 days) neutropenia was consistently noted in those treated with higher doses of the drugs. Nonhematologic effects were uncommon and rarely severe. The pharmacokinetics of 5-FU, assessed in 12 patients on schedule 2, were nonlinear, with the mean area under the time-versus-concentration curve (AUC) increasing from 298 ± 44 to 962 ± 23 µmol/L and mean clearance decreasing from 34 ± 4 to 15.6 ± 0.38 L/h/m2 as the dose of 5-FU was increased from 1,250 to 1,950 mg/m2/wk. 5-FU AUCs achieved with 5-FU 1,250 mg/m2/wk for 6 weeks along with the intensified PN401 dose schedule were approximately five-fold higher than those achieved with 5-FU alone. Plasma uridine concentrations increased with each of the three PN401 doses given every 2 hours, and uridine steady-state concentrations were greater than 50 µmol/L. CONCLUSION: Treatment with oral PN401 beginning 8 hours after 5-FU administration is well tolerated and results in sustained plasma uridine concentrations above therapeutic-relevant levels. The recommended 5-FU dosage for phase II evaluations is 1,250 mg/m2/wk for 3 weeks every 4 weeks with the intensified PN401 dose schedule (schedule 2). At this dose, systemic exposure to 5-FU as measured by AUC was five-fold higher than that observed after administration of a conventional 5-FU bolus.
FLUOROURACIL (5-FU) IS one of the most widely used antineoplastic agents and the mainstay of chemotherapy for gastrointestinal and other types of cancers.1 The principal mechanisms of 5-FU cytotoxicity include inhibition of thymidylate synthase, largely through the actions of its metabolite, fluorodeoxyuridine monophosphate (FdUMP); and inhibition of RNA synthesis as a result of incorporation of a second metabolite, fluorouridine triphosphate (FUTP), into RNA.2 The principal toxicities of 5-FU are leukopenia, mucositis, diarrhea, and hand-foot syndrome, with the latter two adverse effects predominating when 5-FU is administered as a continuous intravenous (IV) infusion.3 Like other conventional cytotoxic antineoplastic agents, 5-FU has a relatively narrow therapeutic index, in that toxicity often limits the dose of 5-FU that can be administered, as well as its overall therapeutic usefulness. Uridine, a naturally occurring pyrimidine nucleoside, selectively reduces incorporation of FUTP into the RNA of hematopoietic progenitor and gastrointestinal mucosal cells, thereby preventing 5-FU toxicity in normal tissues.4-8 In mice, administration of uridine after treatment with 5-FU reduces toxicity to normal tissues, permits substantial 5-FU dose escalation, and increases overall efficacy of 5-FU.4-8 Results of preclinical and clinical studies indicate that uridine concentrations of at least 50 µmol/L are sufficient to confer protection to normal tissues from the toxic effects of 5-FU.6 Differences in uptake and use of uridine between tumor and normal tissues lie behind uridine’s ability to reduce the toxicities of 5-FU without proportionally reducing antitumor activity.3 Both hematopoietic progenitors and gastrointestinal mucosa stem cells efficiently incorporate exogenous uridine (salvage pathway), whereas most other tissues, including malignant tumors, favor the de novo pathway of uridine nucleotide biosynthesis, in which free uridine is not an intermediate.3 Although uridine has also been demonstrated to protect against 5-FU toxicity in humans, its low and erratic oral bioavailability and the requirement for central venous access for parenteral administration preclude clinical utility.9-12 PN401 (2',3',5'-tri-O-acetyluridine; Pro-Neuron, Inc, Gaithersburg, MD) (Fig 1) is an orally active prodrug of uridine that is efficiently absorbed from the gastrointestinal tract and deacetylated by nonspecific esterases, yielding uridine and free acetate. In contrast to oral uridine, PN401 is not a substrate for the catabolic enzyme uridine phosphorylase and does not require the pyrimidine transporter for absorption. Consequently, administration of PN401 results in substantially more bioavailable uridine than does oral administration of uridine itself. In an earlier phase I study of 5-FU given weekly in combination with PN401, in which PN401 was administered every 6 hours for 10 doses beginning 24 hours after 5-FU administration, PN401 doses as high as 9.9 g were well tolerated.13 Treatment with PN401 alone increased plasma uridine concentrations from pretreatment levels ranging from 3 to 6 µmol/L to peak concentrations averaging 167.6 ± 36.9 µmol/L. After multiple doses of PN401, uridine trough concentrations averaged 67.1 ± 19.1 µmol/L. Plasma uridine concentrations readily exceeded 50 µmol/L (the concentration that has consistently been demonstrated to protect normal tissues from the toxic effects of 5-FU) for more than 6 hours.13 In that study, PN401 at its recommended dose of 6 g substantially reduced the incidence and severity of 5-FU toxicity, permitting an increase of the 5-FU dose to 1,000 mg/m2/wk for 6 consecutive weeks.13
Preclinical studies have indicated that timing of the first dose of PN401 relative to 5-FU administration is an important determinant of the effectiveness of PN401 in ameliorating the adverse effects of 5-FU. For example, treatment of mice bearing colon 26 tumors with PN401 2 hours after 5-FU administration resulted in substantially greater antitumor efficacy and less toxicity compared with treatment of mice with PN401 24 hours after 5-FU administration.4 These results suggest that the clinical utility of PN401 can be optimized by administering the agent soon after treatment with 5-FU. Therefore, in this phase I pharmacokinetic study, we evaluated the feasibility of administering high doses of 5-FU on a weekly schedule in combination with PN401, administered starting 8 hours after treatment with 5-FU, unlike in previous investigations in which PN401 was given 24 hours after 5-FU administration.13 Additionally, because of findings that plasma uridine concentrations exceed biologically relevant levels of 50 µmol/L for more than 6 hours, PN401 was initially administered every 8 hours instead of at 6-hour intervals.13 The principal objectives of the present study were to determine the maximum-tolerated dose (MTD) of 5-FU administered as a 30-minute IV infusion weekly for 3 weeks every 4 weeks with PN401 and to recommend doses of these agents for subsequent phase II trials; to characterize the principal toxicities of the regimen; to describe the pharmacokinetic behavior of 5-FU administered with PN401 in this schedule; and to obtain preliminary evidence of antitumor activity, if any.
Eligibility Patients with histologically documented advanced solid malignancies refractory to conventional therapy or for whom no effective therapy existed were candidates for this study. Eligibility criteria included age  18 years; Eastern Cooperative Oncology Group performance status  2; life expectancy greater than 12 weeks; no prior chemotherapy or radiation therapy within 4 weeks of entering onto the study (6 weeks for nitrosoureas and mitomycin); adequate hematopoietic function (WBC count 3,500/µL, absolute neutrophil count [ANC] 1,500/µL, and platelet count 100,000/µL), hepatic function (total bilirubin level 1.5 mg/dL and AST and ALT levels < two times the upper limit of normal [< five times the upper limit of normal for patients with liver metastasis]), and renal function (creatinine level 1.5 mg/dL); no active infection or other coexisting medical problems severe enough to limit compliance; no malabsorption syndrome or other condition that might interfere with intestinal absorption; and documentation of tolerance to treatment with 5-FU or peripheral-blood mononuclear-cell dihydropyrimidine dehydrogenase (DPD) activity of at least 0.12 nmol/min/mg protein. Before treatment, all patients gave written informed consent according to federal and institutional guidelines.
Dosage and Drug Administration 5-FU, administered IV over 30 minutes, was given weekly for 3 weeks every 4 weeks. PN401 was administered at a fixed oral dose starting 8 hours after 5-FU administration. Initially, patients received 6 g of PN401 (12 tablets) every 8 hours for eight doses (schedule 1). After dose-limiting toxicity (DLT) was noted on this schedule, the PN401 dose schedule was modified to 6 g every 2 hours for three doses followed by 6 g every 6 hours for 15 doses (schedule 2). Patients were instructed to take PN401 with water and to repeat the dose if they vomited within 2 hours after ingestion. Patients were asked to record the precise times and numbers of PN401 pills taken in diaries that were collected and reviewed after each course. The starting dose of 5-FU was 1,000 mg/m2. This dose was increased by 25% increments in groups of new patients if the previous dose level was well tolerated. One new patient was treated at each successively higher dose level unless toxicity of at least grade 2 severity was experienced during the first course of treatment. In the event of toxicity of at least grade 2 severity, a minimum of two additional new patients were entered at that dose level. If none of the three patients experienced DLT during course 1, dose escalation resumed, with a minimum of three new patients treated at each successive dose level. If DLT was observed during the first course of treatment in any patient at any dose level, as many as six new patients were treated at that dose level. The MTD and recommended phase II dose were defined as the highest 5-FU dose at which fewer than two of six new patients experienced DLT during course 1. DLT was defined as grade 3 nonhematologic toxicity (excluding nausea or vomiting), any grade 4 nonhematologic toxicity, grade 4 thrombocytopenia (platelet count < 25,000/µL), severe anemia (hemoglobin level < 6.5 g/dL), and grade 4 neutropenia (ANC < 500/µL) lasting more than 5 days and/or associated with fever. Toxicities were graded using the National Cancer Institute common toxicity criteria.14 The weekly dose of 5-FU was reduced by one dose level in patients who experienced either grade 2 nonhematologic or grade 2 or 3 hematologic toxicity on the day of scheduled treatment. 5-FU was not administered to patients who experienced grade 4 hematologic toxicity or grade 3 or 4 nonhematologic toxicity on the day of planned treatment, and the omitted dose was not made up. When treatment was resumed, the dose of 5-FU was reduced by one level for the remainder of treatment. Treatment delays that exceeded 2 weeks because of failure to return to a grade 0 or 1 toxicity level mandated a 5-FU dose reduction by one level for the remainder of treatment unless the toxicity recurred, in which case a second dose reduction was required. The dose of PN401 was not modified in cases of toxicity.
Pretreatment and Follow-Up Studies
Plasma Sampling and Assay The blood samples collected for 5-FU analysis were centrifuged immediately after collection and stored at -20°C until analysis. Plasma (250 µL), thawed from -20°C, was transferred to a 16 x 125-mm silicon tube, and 30 µL of 5-bromouracil, which served as an internal standard, was added. To precipitate proteins, we added 500 mL of 10 mmol/L ammonium sulfate solution, followed by 4 mL of ethyl acetate and isopropanol (90/10 [vol/vol]). The mixture was vortexed for 45 minutes, and the tube was centrifuged for 5 minutes. The organic layer was pipetted into a 13 x 100-mm disposable Pyrex culture tube and dried in a 50°C water bath. The extraction procedure was repeated twice, and each tube was reconstituted with 200 µL of 0.01 mol/L potassium phosphate at pH 4, which was the mobile phase for high-performance liquid chromatography (HPLC). HPLC was performed using a Spectra-Physics Isochrome pump (Spectra-Physics, Mountain View, CA) connected to a Hitachi AS4000 autoinjector (Hitachi, San Jose, CA) and an SP8490 variable-wavelength fluorescence detector (Spectra-Physics) set at 260 nm. After injection of a 20-µL sample, 5-FU and 5-bromouracil were separated using a YMC-Pack, ODS-AMQ (4.6 x 250-mm, 5-µm) C18 column (Yamamura Chemical Co, Kyoto, Japan) using a mobile phase of 0.01 mol/L potassium phosphate, pH 4, and a flow rate of 2.0 mL/min. The retention times of 5-FU and 5-bromouracil were 3.43 and 7.5 minutes, respectively. The standard curve for 5-FU was prepared over a range of 0.246 to 999.4 µmol/L by adding known amounts of 5-FU and internal standard to appropriate volumes of human plasma. A calibration curve was generated by linear regression of the peak height ratio of the 5-FU concentration to that of the internal standard. The values for the intraday and interday precision for this method were less than 2%, and the lower limit of quantification of 5-FU was 0.2 µmol/L based on the extraction of 250 mL of plasma. Plasma uridine concentrations were determined using reverse-phase HPLC that was validated using inosine as an internal standard. Blood samples were centrifuged immediately after collection and stored at -20°C until analysis. Plasma (500 µL), thawed from -20°C, was transferred to a 1.5-mL siliconized centrifuge tube containing 300 mL of water. An aliquot of 20 µL of the internal standard (600 µg/mL inosine in 10% methanol) was added to all control samples, followed by 200 µL of 40% trichloroacetic acid. The tubes were then vortexed completely using a vortex mixer. The sealed tube was next placed in an ice-water bath for 10 to 30 minutes to precipitate plasma proteins. The trichloroacetic acid was extracted with 4 mL of methyl-tert-butyl ether, and the top ether layer was removed. Two hundred microliters of the underlayer was placed in an autosampler microvial. Analysis of the extracted samples was performed on a HPLC system that consisted of a Spectra-Physics Isochrome pump (Spectra-Physics) connected to a Hitachi AS4000 autoinjector (Hitachi) and an SP8490 variable-wavelength fluorescence detector (Spectra-Physics) set at 260 nm. After injection of 10 to 50 µL of sample, uridine was separated by a Kromasil (4.6 x 250-mm, 5-µm) C18 column (Eka Nobel, Bohus, Sweden) using a mobile phase consisting of a mixture of 20 mmol/L sodium acetate at pH 4.5 and 5% methanol at a flow rate of 1.0 mL/min. The retention times of uridine and inosine were 6 and 11 minutes, respectively. The standard curve for uridine was prepared over a range of 0.5 to 1,200 µmol/L by adding known amounts of uridine and internal standard to appropriate volumes of human plasma. A calibration curve was generated by linear regression of the peak height ratio of uridine to the internal standard versus uridine concentration. The values of intraday and interday precision for this method were less than 2%, and the lower limit of quantification of uridine was 1.31 µmol/L based on the extraction of 500 µL of plasma.
Pharmacokinetic and Pharmacodynamic Analysis The peak plasma uridine concentrations after PN401 administration were determined by visual inspection of the uridine concentration–versus-time curves. The mean plasma uridine concentration was calculated as the arithmetic mean of the plasma uridine concentration values obtained for each patient after administration of the first three doses of PN401. Plasma uridine concentrations after administration of PN401 were compared with predicted values obtained by applying the principle of superposition to the plasma concentration–versus-time curve of uridine observed in healthy subjects after a single 6-g oral dose of PN401.15 With this method, the AUC, Cl, Vd, half-life, and ke of uridine after treatment with a single 6-g oral dose of PN401 in healthy volunteers are estimated by a one-compartment open model with first-order absorption kinetics using the program WinNonlin (Statistical Consultants). The estimated parameters are subsequently used to simulate the plasma concentration–versus-time curve of doses of 6 g of PN401 every 2 hours. It is assumed that the pharmacokinetic behavior of uridine after each dose of PN401 is not affected by other doses and that the absorption, conversion of PN401 to uridine, and systemic clearance of uridine do not change with repetitive dosing.
We explored the relationships between toxicity and pharmacokinetic parameters reflecting systemic exposure to both 5-FU and uridine. The percentage decrements in ANCs and platelet counts, as well as the occurrence of DLT, were related to the dose, AUC0-
Twenty-three patients, whose characteristics are listed in Table 1, were treated with a total of 50 courses of 5-FU and PN401 through four dose levels (Table 2). One patient discontinued therapy in the middle of the first course of 5-FU 1,250 mg/m2 and PN401 and was considered unassessable. Twenty-one patients had received previous chemotherapy and 14 patients had previously been treated with 5-FU–containing regimens. The median number of courses administered per patient was two (range, one to six). Four patients required either a reduction (one patient) or omission (three patients) of the weekly dose of 5-FU, and doses were reduced in six patients because of intolerable toxicity in the prior course. Seven patients required 1-week delays in treatment, and a single patient required a 2-week delay because of incomplete recovery of blood cell counts. Major objective antitumor responses were not observed.
PN401 was well tolerated on both dose schedules, and no adverse effects were directly attributed to this agent. On the basis of reviews of patient diaries and interval histories, there was no evidence of noncompliance with PN401 treatment. The rate of DLTs, particularly adverse gastrointestinal and hematologic toxicities, was unacceptably high at the 5-FU dose level of 1,250 mg/m2/wk on the less intensive PN401 dose schedule (schedule 1). Intensification of the PN401 dose schedule (schedule 2) permitted escalation of the dose of 5-FU to 1,950 mg/m2/wk. However, the rate of dose-limiting hematologic toxicities was unacceptable at 5-FU doses greater than 1,250 mg/m2/wk.
Hematologic Toxicity
There were no episodes of DLT in patients treated at the initial dose level in which 5-FU 1,000 mg/m2/wk was administered with PN401 (schedule 1). At the second dose level (5-FU 1,250 mg/m2/wk), however, two of six new patients experienced DLT during course 1. These events included grade 3 diarrhea, in one patient, and grade 4 neutropenia complicated by sepsis and death, in the other patient, a 67-year-old heavily pretreated patient who had advanced pancreatic carcinoma, an Eastern Cooperative Oncology Group performance status of 1, and a normal DPD level. Another heavily pretreated individual developed grade 4 neutropenia associated with fever during courses 2 and 3. With the occurrence of these toxicities at the second dose level of 5-FU on schedule 1, the PN401 dose schedule was intensified (schedule 2). On schedule 2, 5-FU was better tolerated and none of the three new patients treated with 5-FU 1,250 mg/m2/wk experienced DLT. Hematologic toxicity was moderate and consisted of grade 2 neutropenia (in two courses), grade 3 neutropenia (in one course), and grade 2 thrombocytopenia (in one course). Therefore, the doses of 5-FU were successively escalated with PN401 (schedule 2) to 1,550 mg/m2/wk and 1,950 mg/m2/wk, which resulted in progressively lower ANC and platelet count nadirs. At the dose level of 5-FU 1,950 mg/m2/wk, on PN401 dose schedule 2, median ANC and platelet count nadirs were 205/µL and 38/µL, respectively. In addition, both minimally pretreated patients who received 5-FU 1,950 mg/m2/wk and PN401 (schedule 2) experienced prolonged (> 5 days) grade 4 neutropenia and grade 3 thrombocytopenia. Therefore, additional patients were treated at the next-lower dose level, 5-FU 1,550 mg/m2/wk, along with PN401 (schedule 2), and two of three new patients experienced DLT. In both subjects who developed DLT, including a heavily pretreated patient and a previously untreated patient, ANC nadirs decreased to less than 500/µL for more than 5 days. The heavily pretreated individual also developed grade 3 thrombocytopenia. On the basis of these results, two additional new patients were treated with 5-FU 1,250 mg/m2/wk on PN401 dose schedule 2. Overall, none of five new patients, of whom three were heavily pretreated, developed DLT at this dose level.
Nonhematologic Toxicity
Pharmacologic Studies Thirteen patients treated with 5-FU and PN401 (schedule 2) underwent complete plasma sampling for pharmacokinetic studies of 5-FU and uridine. Technical difficulties with the assay precluded use of plasma concentration data from one subject. Plasma 5-FU concentrations peaked at the end of infusion and were less than 5 µmol/L at the last sampling time, 3 hours after infusion. The relationship between Cmax and dose was linear (r2 = .71, P = .005), with Cmax values increasing from 375 ± 81 to 842 ± 7 µmol/L as the dose of 5-FU increased from 1,000 to 1,950 mg/m2/wk. Table 5 lists pertinent 5-FU pharmacokinetic parameters derived using noncompartmental methods. The pharmacokinetics appeared to be dose dependent; 5-FU Cl decreased from 34 ± 3.94 L/h/m2 at the 1,250 mg/m2/wk dose level to 15.6 ± 0.38 L/h/m2 at the 1,950 mg/m2/wk dose level (P = .006), and 5-FU AUC0-  increased disproportionally from 298 ± 44 µmol/L/h at the 1,250 mg/m2/wk dose level to 962 ± 23 µmol/L/h at the 1,950 mg/m2/wk dose level.
Plasma uridine concentrations increased with each successive 2-hour dose of PN401 to a mean Cmax of 259.33 µmol/L 2 hours after the third dose of PN401. Thereafter, uridine concentrations were sustained above 100 µmol/L for more than 6 hours, at which time a fourth dose was administered (Fig 2). There was substantial interindividual variability in plasma uridine concentrations in the 5-FU dose range of 1,250 to 1,950 mg/m2/wk (Fig 2), and no relationship between plasma uridine concentration and 5-FU dose was apparent. The plasma uridine concentrations after treatment with high doses of PN401 in this study were satisfactorily predicted by applying the principle of superposition to the pharmacokinetic data from healthy volunteers who were each treated with a single 6-g dose of PN401, as illustrated in Fig 3.
Pharmacodynamic relationships between 5-FU Cmax and AUC and principal toxicities of 5-FU were evaluated. With regard to the effects of 5-FU and PN401 on neutrophils and platelets, interindividual variability in the percentage decrements in ANCs and platelet count was marked, and these relationships could be described with neither linear nor nonlinear models. In addition, the propensity to develop DLT was not related to peak plasma 5-FU concentrations, as demonstrated by similar Cmax values in patients who did and those who did not experience DLT (506 ± 180 v 682 ± 189 µmol/L; P = .2 [Mann-Whitney U test]). In contrast, the mean AUC0- among patients who developed DLT (703 ± 300 µmol/L) was higher, albeit not significantly, than that among patients who did not (458 ± 210 µmol/L) (P = .1 [Mann-Whitney U test]). Similarly, patients who did and those who did not develop DLT had similar maximum plasma uridine concentrations (285 ± 73 and 267 ± 74 µmol/L, respectively; P = .4 [Mann-Whitney U test]) and mean plasma uridine concentrations (186 ± 33 and 178 ± 38 µmol/L, respectively; P = .99 [Mann-Whitney U test]).
PN401, an oral prodrug of uridine, is being developed to reduce the incidence and severity of the toxicities associated with administration of 5-FU and thereby allow 5-FU dose escalation. Although 5-FU is administered on a broad range of dose schedules and no single dose-schedule has ever emerged as clearly superior, a rapid IV infusion, weekly schedule has demonstrated efficacy in several disease settings and is one of the most commonly used schedules.1-3 By protecting hematopoietic progenitors and the gastrointestinal mucosa, PN401 may reduce the incidence and severity of unpredictable toxicities in patients receiving 5-FU and confer tolerance of high doses, resulting in greater drug exposure compared with conventional dose schedules.13 Preclinical and prior clinical studies have demonstrated that uridine protects against the nonhematologic and hematologic toxicities of 5-FU, presumably by competing with FUTP for incorporation into the RNA of normal tissues.4-12 In preclinical studies, the therapeutic index of 5-FU has been shown to be improved; animals treated with both 5-FU and PN401 tolerate much higher doses of 5-FU, which induce less toxicity and a greater degree of tumor regression compared with 5-FU alone.4 However, uridine is inherently toxic and its administration is cumbersome.9-12 IV administration requires the use of a central venous catheter to avoid phlebitis, whereas oral uridine is poorly and erratically absorbed, requiring the administration of high doses, which often result in severe diarrhea.9-12 PN401 is an acetylated prodrug of uridine that is more lipophilic, resulting in enhanced transport across the gastrointestinal mucosa. Whether PN401 or any other biochemical modulator will augment the therapeutic indices of the fluoropyrimidines will depend on the relative modulating effects of these agents in malignant tumors and normal tissues.3 Preclinical studies in animal tumors have demonstrated that PN401 enhances the therapeutic index of 5-FU, provided that the agent is administered on dose schedules that result in plasma uridine concentrations of approximately 50 µmol/L, a level that is substantially higher than basal concentrations in humans (3 to 6 µmol/L).6,13 In an early phase I study of PN401 and 5-FU, PN401, given in 6-g doses every 6 hours for a total of 10 doses beginning 24 hours after treatment with 5-FU, enabled safe administration of 5-FU in the form of rapid IV infusion at doses as high as 800 mg/m2/wk for 6 weeks, which was the recommended dose for phase II studies.13 Plasma uridine concentrations exceeded 50 µmol/L during PN401 treatment in that study. In the current study, we sought to optimize the biochemistry-modulating potential of PN401 by introducing two principal modifications in the PN401 dose schedule that was investigated in the early phase I study.13 First, on the basis of the results of preclinical studies indicating that high doses of 5-FU are better tolerated and more efficacious when the interval between 5-FU and PN401 administration is shorter than 24 hours, the current study was designed so that treatment with PN401 commenced 8 hours after 5-FU administration. The second modification was the use of an 8-hour interval between doses of PN401 (schedule 1); in the earlier phase I study, plasma uridine concentrations exceeded 50 µmol/L for more than 6 hours after a 6-g dose. In the current study, the interval was subsequently reduced to 2 hours for the first three doses and 6 hours thereafter (schedule 2) to enhance the protective effects of PN401 after DLT consistently occurred in patients treated with 5-FU and on PN401 dose schedule 1.13 The results of this study demonstrate that PN401 protects against the principal toxic effects of 5-FU, confirming the results of the earlier phase I study.13 In contrast to uridine, PN401 was well tolerated. In fact, the qualitative and temporal natures of the toxicities of the 5-FU–PN401 regimens indicate that PN401, itself, does not induce clinically significant toxicity. The toxicities of the 5-FU–PN401 regimens evaluated in the present study were qualitatively similar to those associated with 5-FU alone on a weekly schedule, with myelosuppression predominating.1,3 The principal DLT was severe (grade 4) neutropenia that was protracted (> 5 days). Severe neutropenia consistently occurred in patients treated with 5-FU doses exceeding 1,000 mg/m2/wk and on PN401 dose schedule 1 and in patients treated with 5-FU doses exceeding 1,250 mg/m2/wk and on PN401 dose schedule 2. Severe nonhematologic effects were uncommon, even when relatively high doses of 5-FU were administered weekly for 6 weeks. Overall, the safe administration of 5-FU on a weekly schedule in the dosing range of 1,000 to 1,250 mg/m2/wk, which is at least two-fold higher than the MTD of 5-FU without biochemical modulation, substantiates the potent modulating capabilities of PN401.1-3,13 However, considering the relatively low number of patients treated at the MTD of 5-FU and PN401 on this study and the unpredictable and sometimes overwhelming toxicities of 5-FU in the individual patients, further studies are required to elucidate fully the safety and tolerability of this combination. The potential of PN401 to protect against the toxicities of 5-FU, as well as the dose- and schedule-dependent nature of these effects, was further demonstrated as the PN401 dose schedule was intensified. With the more intensive PN401 dose schedule (schedule 2), plasma uridine concentrations progressively increased after each successive dose of PN401 given at 2-hour intervals, and plasma uridine concentrations were sustained above 100 µmol/L for most of the PN401 treatment period in most patients. This more intensive schedule permitted 5-FU doses to be increased by an additional 25% over the MTD established for 5-FU and PN401 on schedule 1. In addition, the highest safest dose of 5-FU achieved in the current study with PN401 rescue initiated 8 hours after 5-FU was 56% higher than the highest safest 5-FU dose achieved in the earlier study in which there was a 24-hour treatment interval.13 Although the pharmacokinetics of 5-FU were not assessed in patients treated with 5-FU and PN401 on schedule 1, the nonlinear pharmacokinetics of 5-FU, as demonstrated in patients receiving 5-FU and PN401 on schedule 2 and in patients treated with 5-FU without biochemical modulation, indicate that the two-fold increase in the MTD of 5-FU due to PN401 itself, and the additional 25% increase afforded by the more intensive PN401 dose schedule, resulted in a disproportionally greater increase in 5-FU exposure. At the recommended phase II dosage of 5-FU, 1,250 mg/m2/wk and the intensified PN401 dose schedule, 5-FU AUCs were approximately five-fold higher than those achieved without biochemical modulation.16-21 Although the results of preclinical studies suggest that the inhibition of thymidylate synthase by FdUMP is saturated in the range of drug exposure achieved with conventional 5-FU dose schedules without biochemical modulation, the magnitude of 5-FU–induced cytotoxicity due to the incorporation of the 5-FU anabolite, FUTP, into RNA in vitro may not be saturated,3 and this potentially important mechanism of cytotoxicity might not be taken full advantage of in standard dose-schedules. Although the PN401 dose schedules used in the current study were selected to simulate closely those that were efficacious in preclinical studies in animals, still unknown is the optimal timing of PN401 or uridine treatment relative to administration of 5-FU to protect normal tissues maximally without protecting malignant tissue. It is of some concern that antitumor activity was not observed in the current study, despite the preponderance of patients with gastrointestinal malignancies. However, the disease of most of these patients had been demonstrated to be refractory to 5-FU treatment, with progressive tumor growth occurring during prior treatment. The 8-hour interval between treatment with 5-FU and PN401 in this study derived from findings of studies in murine colon 26 tumors.13 In these studies, the MTD of 5-FU given weekly for 3 weeks was 100 mg/kg, which inhibited the growth of 60% of tumors, but there were no complete tumor regressions.13 However, treatment with oral PN401 2 hours after 5-FU administration resulted in an MTD for 5-FU of 200 mg/kg and a high incidence of complete tumor regression. On the other hand, the MTD was 150 mg/kg when treatment with PN401 was initiated 24 hours after 5-FU administration; antitumor activity was superior to that observed with treatment with 5-FU alone, but complete tumor regressions were not observed.13 These results support the use of shorter treatment intervals between 5-FU and PN401, such as the 8-hour interval used in the current study. However, the optimal timing between administration of 5-FU and that of PN401 is not known, and further evaluations are required to elucidate this seemingly important facet of 5-FU–PN401 administration. In addition, although the use of PN401 and other uridine analogs for the sole purpose of increasing doses of 5-FU may not be appropriate, because the superiority of high doses of 5-FU (1,000 to 1250 mg/m2/wk) over conventional doses without rescue (500 to 600 mg/m2/wk) has not been firmly established in randomized trials, the principal utility of uridine-based rescue may be prevention or amelioration of the toxic effects of 5-FU in conventional-dose regimens, as well as conferring of tolerance to patients who cannot tolerate conventional 5-FU doses and would otherwise require dose reduction. The utility of PN401 in this regard is further supported by the results of studies indicating that lower doses of 5-FU (approximately 300 mg/m2/wk) are inferior to conventional doses in patients with advanced colorectal cancer.22-24 In summary, the results of this study demonstrate that PN401 treatment consistently results in plasma uridine concentrations exceeding those capable of modulating the actions of 5-FU. It is also clear that the uridine exposure resulting from PN401 greatly protects normal tissues from the toxic effects of 5-FU, as demonstrated by the tolerance of 5-FU doses as high as 1,250 mg/m2 on a weekly schedule, a level that is approximately two-fold higher than maximally tolerated 5-FU doses without biochemical modulation. Because of the nonlinearity of 5-FU pharmacokinetics, this modest increase in the MTD of 5-FU was associated with a five-fold increase in 5-FU exposure (ie, AUC). However, considering the relative low number of patients treated at the recommended phase II dose of 5-FU and PN401 on this schedule, further studies are needed to evaluate fully the safety and tolerability of this regimen. In addition, disease-directed randomized clinical trials must be performed to assess whether biochemical modulation with PN401 enhances the therapeutic indices of 5-FU in relevant disease settings.
Some patients were treated at the Frederic C. Barter Clinical Research Unit of the Audie Murphy Veterans Administration Hospital; that part of the study was supported in part by National Institutes of Health grant no. MO1 RR01346. M.H. was supported in part by grant no. PF 97 52273279 from the Ministerio de Educación y Cultura, Spain, and is the recipient of a National Cancer Institute–European Organization for Research and Treatment of Cancer Fellowship Award. Presented in part at the Thirty-Fourth Annual Meeting of the American Society of Clinical Oncology, Los Angeles, CA, May 16-19, 1998. M.H and M.A.V.C. contributed equally to this work and should both be considered first authors.
1. Allegra CJ, Grem JL: Antimetabolites, in DeVita VT Jr, Hellman S, Rosenberg SA (eds): Cancer: Principles and Practice of Oncology (ed 5). Philadelphia, PA,Lippincott-Raven, 1997, pp 432-451
2.
Sobrero AF, Aschele C, Bertino JR: Fluorouracil in colorectal cancer: A tale of two drugs—Implications for biochemical modulation. J Clin Oncol 15:368-381, 1997 3. Grem JL: 5-Fluoropyrimidines, in Chabner BA, Longo DL (eds): Cancer Chemotherapy and Biotherapy: Principles and Practice. Philadelphia, PA, Lippincott-Raven, 1996, pp 149-211 4. Martin DS, Stolfi RL, Sawyer RC, et al: High-dose 5 fluorouracil with delayed "uridine" rescue in mice. Res 42:3964-3970, 1982 5. Klubes P, Cerna I, Meldon MA: Uridine rescue from the lethal toxicity of 5-fluorouracil in mice. Cancer Chemother Pharmacol 8:17-21, 1982[Medline] 6. Martin DS, Stolfi RL, Sawyer RC, et al: Use of oral uridine as a substitute for parenteral uridine rescue of 5-fluorouracil therapy, with and without the uridine phosphorylase inhibitor 5-benzylacyclouridine. Cancer Chemother Pharmacol 24:9-14, 1989[Medline] 7. Klubes P, Leyland-Jones B: Enhancement of the antitumor activity of 5-fluorouracil by uridine rescue. Pharmacol Ther 41:289-302, 1989[Medline] 8. Nord LD, Stolfi RL, Martin DS: Biochemical modulation of 5-fluorouracil with leucovorin or delayed uridine rescue: Correlation of antitumor activity with dosage and FUra incorporation into RNA. Pharmacol 43:2543-2549, 1992
9.
Leyva A, van Groeningen CJ, Kraal I, et al: Phase I and pharmacokinetic studies of high-dose uridine intended for rescue from 5-fluorouracil toxicity. Cancer Res 44:5928-5933, 1984
10.
van Groeningen CJ, Peters GJ, Nodal JC, et al: Clinical and pharmacologic study of orally administered uridine. J Natl Cancer Inst 83:437-441, 1991 11. Seiter K, Kemeny N, Martin D, et al: Uridine allows dose escalation of 5-fluorouracil when given with N-phosphonacetyl-L-aspartate, methotrexate, and leucovorin. Cancer 71:1875-1881, 1993[Medline] 12. Schwartz GK, Christman K, Saltz L, et al: A phase I trial of a modified, intensive FAMTX regimen (high dose 5-fluorouracil+ doxorubicin+high dose methotrexate+leucovorin) with oral uridine rescue. Cancer 78:1988-1995, 1996[Medline] 13. Kelsen DP, Martin D, O’Neil J, et al: Phase I trial of PN401, an oral prodrug of uridine, to prevent toxicity from fluorouracil in patients with advanced cancer. J Clin Oncol 15:1511-1517, 1997[Abstract] 14. National Cancer Institute:Guidelines for Reporting of Adverse Drug Reactions.Bethesda, MD, Division of Cancer Treatment, National Cancer Institute, 1988 15. Shargel L, Yu ABC: Multiple-dosage regimen, in Shargel L, Yu ABC (eds): Applied Biopharmaceutics and Pharmacokinetics. Norwalk, CT,Appleton & Lange, 1993, pp 353-374
16.
Harris BE, Song R, Soong S, et al: Relationship between dihydropyrimidine dehydrogenase activity and plasma 5-fluorouracil levels with evidence for circadian variation of enzyme activity and plasma drug levels in cancer patients receiving 5-fluorouracil by protracted continuous infusion. Cancer Res 50:197-201, 1990 17. Larsson A, Carlsson G, Gustavsson B, et al: Different intravenous administration techniques for 5-fluorouracil: Pharmacokinetics and pharmacodynamic effects. Acta Oncol 35:207-212, 1996[Medline] 18. McDermott BJ, van der Berg HW, Murphy RF: Nonlinear pharmacokinetics for the elimination of 5-fluorouracil after intravenous administration in cancer patients. Chemother Pharmacol 9:173-178, 1982
19.
MacMillan WE, Wolberg WH, Welling PG: Pharmacokinetics of fluorouracil in humans. Cancer Res 38:3479-3482, 1978
20.
Heggie GD, Sommadossi JP, Cross DS, et al: Clinical pharmacokinetics of 5-fluorouracil and its metabolites in plasma, urine, and bile. Cancer Res 47:2203-2206, 1987
21.
van Groeningen CJ, Pinedo HM, Heddes J, et al: Pharmacokinetics of 5-fluorouracil assessed with a sensitive mass spectrometric method in patients on a dose escalation schedule. Cancer Res 48:6956-6961, 1988
22.
Horton J, Olson KB, Sullivan J, et al: 5-Fluorouracil in cancer: An improved regimen. Ann Intern Med 73:897-905, 1970 23. Leichman L, Fabian C, O’Bryan R, et al: Evaluation of 5-FU: A phase II trial in metastatic adenocarcinoma of the large bowel. Proc Am Soc Clin Oncol 2:120, 1983 (abstr 469) 24. Mayer RJ, McIntyre JM, Steele GD: High dose bolus 5-fluorouracil (5-FU) for metastatic colorectal carcinoma. Proc Am Soc Clin Oncol 2:127, 1983 (abstr 498) Submitted February 22, 1999; accepted August 11, 1999.
This article has been cited by other articles:
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
|
|||||||||||
|
|||||||||||
|
|
Copyright © 2000 by the American Society of Clinical Oncology, Online ISSN: 1527-7755. Print ISSN: 0732-183X
|
TOP
ABSTRACT
INTRODUCTION
), 1,550 (
), and 1,950 mg/m2 (
) and PN401 (schedule 2).
