DX-8951F), Infused Over 30 Minutes Every Three Weeks1

Home , Exatecan, Lurtotecan, Nitrosourea

3056 Vol. 7, 3056–3064, October 2001 Clinical Cancer Research

Phase I and Pharmacological Study of a New Camptothecin Derivative, Exatecan Mesylate (DX-8951f), Infused Over 30 Minutes Every Three Weeks1

Hironobu Minami,2 Hirofumi Fujii, toxicity profile of mild and infrequent diarrhea. Interpatient Tadahiko Igarashi, Kuniaki Itoh, Kyoji Tamanoi, variability of pharmacokinetics was similar to or smaller Toshihiro Oguma, and Yasutsuna Sasaki than that with other camptothecin derivatives. Division of Oncology/Hematology, Department of Medicine, National Cancer Center Hospital East, Kashiwa 277-8577 [H. M., H. F., T. I., INTRODUCTION K. I., Y. S.], and Daiichi Pharmaceutical Co., Ltd., Tokyo 134-8630 Camptothecin derivatives are a unique class of anticancer [K. T., T. O.], Japan agents that have an inhibitory effect on topoisomerase I activity as their mechanism of action (1–3). Among camptothecin derivatives that have been evaluated clinically, irinotecan and ABSTRACT topotecan are the most widely used drugs for which antineo- Purpose: A Phase I study of exatecan, a new water- plastic activities have been clinically confirmed (4). Irinotecan soluble camptothecin derivative, was conducted to deter- in particular has been demonstrated to be superior to conven- mine the maximum tolerated dose and a recommended dose, tional therapies that had been widely used for colorectal cancers according to an internationally standardized core protocol. and now plays an indispensable role in the standard therapy ؉ Pharmacological profiles of lactone and total (lactone (5–7). Furthermore, evidence of the superior activity of irinote- carboxylate) exatecan were also investigated. can over conventional drugs against both small and non-small Patients and Methods: Fifteen patients with advanced 2 cell lung cancers has accumulated (8–10). solid malignancies were treated with 3, 5, and 6.65 mg/m of Severe and unpredictable diarrhea caused by irinotecan, exatecan infused over 30 min every 3 weeks. Concentrations however, has hampered its clinical use. Irinotecan undergoes of lactone, total drug, and a metabolite in plasma and urine metabolic activation by carboxyesterase to yield the active me- were determined during the first course. 3 tabolite, SN-38, which is further inactivated by uridine diphos- Results: Dose-limiting neutropenia and liver dysfunc- phate glucuronosyl transferase (11). Polymorphism in these tion were observed in two of six patients at 6.65 mg/m2 , but enzymes leads to pharmacokinetic interpatient variation of irino grade 3 or worse diarrhea was observed. Emesis was notecan and explains, at least in part, the unpredictable severe moderate, and no grade 3 or worse nausea and vomiting 2 toxicity (12–14). were observed at a recommended dose of 5 mg/m , with Exatecan is a novel synthetic camptothecin derivative with prophylactic use of granisetron. Pharmacokinetics were lin- a unique hexacyclic structure (15). The compound is water ear and had moderate variability; clearances of lactone and soluble and does not require metabolic activation. Similar to 2 ؎ ؎ ؎ total drug were 6.8 2.8 and 2.1 1.1 (mean SD) l/h/m , other camptothecin derivatives, the closed lactone ring of ex- respectively. The ratio of lactone concentration to total drug atecan undergoes pH-dependent hydrolysis to yield an open ؎ concentration in plasma decreased from 0.81 0.06 at the carboxylate form (Fig. 1). An intact lactone ring was found to be ؎ end of infusion to 0.15 0.06 10 h after the infusion. The necessary for topoisomerase I inhibition. Two metabolites, ؎ lactone:total ratio of drug exposure was 0.30 0.08, ranging UM-1 and UM-2, were found in urine after exatecan was in- from 0.16 to 0.43. Neutropenia was related to the drug fused to rats. In vitro experiments using human liver micro- exposure of both lactone and total drug. somes and microsomes from cell lines expressing human CYPs Conclusions: The recommended dose of exatecan in- 2 revealed that UM-1 was the major metabolite in liver micro- fused over 30 min every 3 weeks is 5 mg/m , with a favorable somes and that CYP3A4 was the isozyme responsible for me- tabolizing exatecan to UM-1. The inhibitory effect of exatecan on topoisomerase I activity is 3 and 10 times higher than those of SN-38 and topotecan, Received 1/19/01; revised 7/12/01; accepted 7/12/01. respectively (15, 16). In vitro experiments using various cell The costs of publication of this article were defrayed in part by the lines derived from solid tumors showed that exatecan was 6 and payment of page charges. This article must therefore be hereby marked 28 times more active than SN-38 and topotecan, respectively advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 Supported in part by a Grant-in-Aid for Cancer Research from the Ministry of Health and Welfare and a grant from Daiichi Pharmaceutical Co., Ltd., Japan. 3 The abbreviations used are: SN-38, 7-ethyl-10-hydroxycamptothecin; 2 To whom requests for reprints should be addressed, at Division of exatecan, DX-8951f, (1S,9S)-1-amino-9-ethyl-5-fluoro-1,2,3,9,12,15- Oncology/Hematology, Department of Medicine, National Cancer Cen- hexahydro-9-hydroxy-4-methyl-10H,13H-benzo[de]pyrano[3Ј,4Ј:6,7] ter Hospital East, 6-5-1 Kashiwanoha, Kashiwa, Chiba 277-8577, Japan. indolizino[1,2-b]quinoline-10,13-dione monomethanesulfonate dihydrate; Phone: 81-471-33-1111; Fax: 81-471-34-6922; E-mail: hminami@east. CYP, cytochrome P-450; AUC, area under the time-concentration ncc.go.jp. curve.

Fig. 1 Structures of exatecan (lactone and carboxylate forms) and its metabolites (UM-1 and UM-2). The lactone and carboxylate forms exist at an equilibrium in aqueous solution, depending on pH.

(15). It also exhibited potent activity against in vivo models of tive therapy was available, who were 20–74 years of age, and acute myelogenous leukemia of early and late stages (17). It has had an Eastern Cooperative Oncology Group performance status greater activity against human tumor xenografts than other of 0–2. Prior chemotherapy and radiotherapy had to be com- camptothecin derivatives including irinotecan, topotecan, and pleted at least 4 weeks (6 weeks for nitrosourea, mitomycin C, lurtotecan (GG-211; Ref. 18). The spectrum of in vitro activity carboplatin, and investigational new drugs) before entry into the of exatecan is broad on tumor colony formation from freshly study, and the total field of prior radiotherapy was required to explanted human tumor cells, covering gastric, colon, lung, encompass Ͻ20% of hematopoietic bones. Patients were re- breast, head and neck, ovarian, and prostate cancers and pedi- quired to have a life expectancy of Ն2 months and adequate atric tumors (19). Furthermore, the therapeutic ratio (maximum organ function, with leukocyte count Ն4,000/mm3 and tolerated dose:minimum effective dose) of exatecan was 2–10 Ͻ12,000/mm3, absolute neutrophil count Ն2,000/mm3, hemo- times greater than those of irinotecan and topotecan (18, 20). globin Ն9.0 g/dl, platelet count Ն100,000/mm3, aspartate ami- Finally, exatecan was effective against irinotecan-, SN-38-, and notransferase and alanine aminotransferase Յ2 ϫ normal, bili- topotecan-resistant cell lines (16, 18, 21), and unlike topotecan Յ Ն rubin 1.5 mg/dl, creatinine clearance 50 ml/min, and PaO2 (15, 22), exatecan was effective against a P-glycoprotein-medi- Ն60 mm Hg. Patients with diarrhea, bowel obstruction or pa- ated multidrug-resistant cell line (15). ralysis, interstitial pneumonitis, a history of heart failure or On the basis of these studies showing that exatecan had myocardial infarction within 6 months, or a history of allergy to more potent antitumor activity and less toxicity than other irinotecan or other camptothecin derivatives were ineligible. camptothecin derivatives, Phase I studies with various infusion Patients with a symptomatic brain metastasis, active infection, schedules have been conducted in Japan, the United States, and uncontrolled diabetes mellitus, liver cirrhosis, or pregnancy Europe. Before each Phase I study was planned, a standardized were also excluded from the study. core protocol for Phase I studies was defined, and protocols of This Phase I study was conducted in accordance with the all of the Phase I studies were based on the core protocol, which guidelines of the Ministry of Health and Welfare, Japan. Written standardized eligibility and exclusion criteria, toxicity, and ef- informed consent according to institutional and regulatory re- ficacy criteria, the definition of dose-limiting toxicities and the quirements was obtained from all patients, and the study was maximum tolerated dose, and supportive therapies. In this study approved by the Institutional Review Board of the National in Japan, we conducted a Phase I study of exatecan infused over Cancer Center, Japan. 30 min every 3 weeks to determine the maximum tolerated dose Dosing and Follow-Up. Exatecan was dissolved in nor- and a recommended dose. Pharmacokinetic profiles of the lac- mal saline (100 ml) and infused i.v. over 30 min; the treatment ϩ tone form and total (lactone carboxylate) drug were also was repeated every 3 weeks after patients recovered from the investigated. toxicities. Dogs were found to be more sensitive to exatecan than mice in preclinical toxicological studies, and the starting PATIENTS AND METHODS dose was one-third of the toxic low dose in dogs (3 mg/m2). A Eligibility. Eligibility criteria included patients with his- 100% dose escalation was planned until grade 2 nausea or tologically or cytologically confirmed solid malignant tumors vomiting or other grade 1 toxicities were observed, after which that were refractory to standard therapy or for which no effec- successive dose increments of 65, 50, and 40%, followed by

33% escalation for all subsequent levels, were planned. At least Table 1 Demographic profile of patients 3 patients were treated at one dose level, and the dose was No. of patients 15 increased if none experienced dose-limiting toxicity. When Female:male ratio 8:7 dose-limiting toxicity was observed in 1 of 3 patients, the dose Age (yr) level was expanded to 6 patients, and 33% dose escalations were Median 58 performed thereafter. The dose escalation was continued until 2 Range 40–73 Performance status (0/1) 1/14 of the first 3 patients or 2 of the 6 patients experienced dose- Primary cancer limiting toxicity. Breast 4 Toxicities were graded according to the National Cancer Non-small cell lung 2 Institute Common Toxicity Criteria. The dose-limiting toxicities Head and neck 4 Others 5 included grade 4 neutropenia lasting for 5 days or longer, grade Prior therapy 4 neutropenia complicated by fever (Ն38.5°C) of infection, Chemotherapy 14 grade 4 thrombocytopenia or anemia, and grade 3 or greater Radiotherapy 5 nonhematological toxicity, except for alopecia, nausea, and vomiting. Vomiting was considered to be dose limiting when grade 4 vomiting was observed after prophylactic use of granisetron. The maximum tolerated dose was defined as the highest ing toxicities at 6.65 mg/m2 was found to have an AUC three dose resulting in dose-limiting toxicities in not more than one of times higher than other patients treated at the same dose and a six patients during the first treatment course. Antiemetics were considerable level of the drug remained in plasma 48 h after the not routinely used prophylactically until one patient experienced end of infusion. Therefore, additional blood samplings for phar- grade 2 vomiting, after which granisetron at a dose of 3 mg was macokinetics were performed in this patient at 9 and 13 days used prophylactically in all patients. after the treatment. A complete medical history, physical examination, and For pharmacodynamic analysis of the relationship between tumor measurement were performed and recorded prior to the the AUC and neutropenia, the surviving fraction (SF) of neu- therapy. Baseline values of complete blood cell counts with trophils was calculated by dividing the nadir neutrophil count by differential and platelet counts were obtained, and measure- the pretreatment count. Relationships between the surviving ments were repeated at least twice a week after treatment. fraction versus the AUC of lactone and total exatecan were Values for serum total protein, albumin, total bilirubin, urea analyzed using an inhibitory sigmoid Emax model: nitrogen, creatinine, uric acid, aspartate aminotransferase, alanine aminotransferase, ␥-GTP, alkaline phosphatase, lactate de- AUC r ϭ Ϫ SF 1 r ϩ r hydrogenase, glucose, and electrolytes including sodium, potas- AUC50 AUC sium, chloride, and calcium were obtained prior to the therapy and were tested at least weekly after the treatment. Tumor where r is a sigmoidicity parameter and AUC50 is a parameter measurement was repeated after each course and antitumor corresponding to an AUC value that produced a 50% decrease in responses were assessed using the Southwest Oncology Group neutrophil counts. Correlation between observed SF and SF response criteria. All patients were treated and monitored in a predicted by the model was evaluated using Pearson correlation hospital for the first course. coefficient. Pharmacological Study. Blood sampling for determina- tions of the pharmacokinetics of exatecan was conducted before RESULTS and at 15 min into the exatecan infusion, at the end of infusion, Dose Escalation. Fifteen patients were treated at three 15 and 30 min, and 1, 1.5, 2, 3, 4, 5, 6, 8, 10, 24, and 48 h after dose levels, 3, 5, and 6.65 mg/m2, with a total of 30 courses the end of infusion. Plasma concentrations of the lactone form (median number of courses in each patient was 2, range 1–5). and total (lactone ϩ carboxylate) exatecan and the metabolite Demographic profiles of the patients are listed in Table 1. UM-1 were measured. Urine was sampled before the infusion, Fourteen of 15 patients had previously been treated with chem- and the voided volumes and concentrations of total exatecan and otherapy, with a median of two regimens (range, 1–6). Al- UM-1 in urine were monitored for 24 h after the infusion. though a 100% dose escalation was planned for the first part of The lactone form and the total concentration of exatecan the dose escalation, grade 2 leukopenia, thrombocytopenia, and were measured using high-performance liquid chromatography vomiting and grade 3 anemia and nausea were observed at the methods (23), and high performance liquid chromatography- first dose level. As a result, the dose was increased by 67% to 5 mass spectrometry was used for the measurement of UM-1 mg/m2, at which one of six patients developed dose-limiting concentrations (24). neutropenia and grade 3 thrombocytopenia; the dose was then Pharmacokinetic analysis was performed using a non- increased by 33% to 6.65 mg/m2. At 6.65 mg/m2, two of six compartment method. The pharmacokinetics of the lactone form patients experienced dose-limiting toxicities, grade 3 liver dys- and total exatecan in plasma were calculated separately. The function with dose-limiting neutropenia in one patient, and elimination constant was calculated by linear regression of the grade 3 liver dysfunction in another. On the basis of these terminal log-linear part of the concentration-time curves. AUC observations, the maximum tolerated dose was determined to be was calculated using the linear trapezoidal rule from time zero 5 mg/m2. to the last point measured and extrapolated to infinity using the Hematological Toxicity. The worst hematological toxic- elimination constant. One patient who experienced dose-limit- ities observed in all courses of each patient are listed in Table 2.

Table 2 Worst hematological toxicity per patient across all courses Leukopenia Neutropenia Thrombocytopenia Anemia Dose No. of No. of Grade Grade Grade Grade (mg/m2) patients courses 1/2/3/4 1/2/3/4 1/2/3/4 1/2/3/4 3 3 4 1/1/0/0 2/0/0/0 0/1/0/0 0/2/1/0 5 6 16 1/2/1/0 1/1/1/1a 1/0/1/0 2/3/0/0 6.65 6 10 0/3/1/1 1/2/1/1a 3/0/1/0 2/3/1/0 a Dose-limiting toxicity.

Table 3 Worst nonhematological toxicity per patient across all courses Nausea Vomiting Diarrhea Liver Skin Dose No. of No. of Grade Grade Grade Grade Grade (mg/m2) patients courses 1/2/3/4 1/2/3/4 1/2/3/4 1/2/3/4 1/2/3/4 3 3 4 0/2/1/0 1/1/0/0 0/0/0/0 0/0/0/0 0/0/0/0 5 6 16 3/3/0/0 2/2/0/0 1/1/0/0 0/1/0/0 0/2/0/0 6.65 6 10 2/2/2/0 1/2/2/0 1/0/0/0 0/0/2a/0 0/0/0/0 a Dose-limiting toxicities.

Fig. 2 Time courses of lactone and total exatecan and a metabolite (UM-1) in a representative patient.

Neutropenia was the principal hematological toxicity, and dose- Fig. 3 Time course of the ratio of lactone concentration to total con- limiting neutropenia was observed in the first course of two centration. Means of the ratio in all patients are shown; bars, SD. patients. One of six patients treated at 5 mg/m2 developed grade 4 neutropenia lasting for 5 days. At the 6.65 mg/m2 dose level, one patient had AUC that was three times higher than the other Nonhematological Toxicity. Nonhematological toxici- patients treated at the same dose and developed grade 4 neutro- ties for all courses are summarized in Table 3. Nausea and penia, complicated by fever. Granulocyte-colony stimulating vomiting as a result of treatment with exatecan in this schedule factor was given after dose-limiting neutropenia was docu- were moderate to severe. All patients treated at 3 mg/m2 expe- mented, but grade 4 neutropenia lasted for 7 days in this patient. rienced grade 2 or 3 nausea or vomiting, and granisetron was Grade 3 thrombocytopenia was observed in the two patients infused i.v. prior to the administration of exatecan in all patients who experienced dose-limiting neutropenia. The nadir of neu- treated at 5 and 6.65 mg/m2. Grade 3 nausea and vomiting was tropenia in all patients was observed between 4 and 22 (median, observed in two patients at 6.65 mg/m2, but they were grade 2 11) days after the treatment, and for thrombocytopenia the nadir or milder at 5 mg/m2. Although diarrhea was one of the dose- was between 7 and 12 (median, 10) days after the treatment. limiting toxicities of irinotecan, diarrhea with exatecan was mild Neutrophil counts recovered to 2000/␮lby14–37 (median, 24) and infrequent. Grade 3 or greater diarrhea was not observed, days after the treatment, and platelet counts recovered to and only one episode of grade 2 diarrhea was observed at 5 100,000/␮lby13–17 (median, 15) days after the treatment. At mg/m2. Asymptomatic grade 3 liver dysfunction with elevations a recommended dose of 5 mg/m2, treatment at 3-week intervals of serum levels of bilirubin and transaminases was observed in was possible in five of six patients, and the treatment was the first course of 2 patients at 6.65 mg/m2. The peak levels of repeated every 4 weeks in one patient. bilirubin were 2.7 and 1.7 mg/dl, and those for transaminases

Fig. 4 Relationship between the dose of exatecan versus the

AUC of lactone (AUClactone) and the total drug (AUCtotal). The AUClactone and AUCtotal increased linearly with dose escalations.

were 8–11 times the upper limit of the normal range; they were and the total drug (AUCtotal) were investigated. Neutropenia observed 2 or 3 days after the treatment. The levels of bilirubin expressed as the surviving fraction of neutrophils (SF) was 2 ϭ ϭ and transaminases decreased to grade 1 or better by 7 days after significantly related to the AUClactone (r 0.50, P 0.003) 2 ϭ ϭ the treatment and normalized in the second week. Two patients and AUCtotal (r 0.40, P 0.012) as shown in Fig. 5. These

experienced minor and transient skin eruptions with itching relationships were described by inhibitory sigmoid Emax mod- ϭ Ϫ 0.94 0.94 ϩ 0.94 sensations. Although clinically insignificant microscopic hema- els: SF 1 AUClactone /(523 AUClactone ) and ϭ Ϫ 0.65 0.65 ϩ 0.65 turia was observed in one patient, no other sign of hemorrhagic SF 1 AUCtotal /(1518 AUCtotal ). When the cystitis was observed. patient who had the highest AUC was excluded from the phar- Pharmacology. A pharmacokinetic investigation was macodynamic analysis, the relationship with AUClactone re- performed in all patients treated in this study. Time courses of mained significant (r2 ϭ 0.29, P ϭ 0.047), whereas that with the lactone and total exatecan and a metabolite (UM-1) in a repre- 2 ϭ ϭ AUCtotal was no longer significant (r 0.16, P 0.16). When 2 sentative patient treated at 5 mg/m are shown in Fig. 2. The the relationships between the surviving fraction of neutrophil second peak of the concentration-time curve for exatecan was counts and AUC were evaluated using a linear regression model, observed 5–8 h after the treatment in 10 patients. The ratio of 2 ϭ ϭ correlations to AUClactone (r 0.50, P 0.003) and AUCtotal lactone concentration to total concentration decreased from (r2 ϭ 0.44, P ϭ 0.007) were stronger than that to the dose of 0.81 Ϯ 0.06 (mean Ϯ SD) at the end of infusion to 0.15 Ϯ 0.06 exatecan (r2 ϭ 0.19, P ϭ 0.10). With regard to liver dysfunc- 10 h after the infusion and did not change thereafter (Fig. 3). tion, another dose-limiting toxicity in this study, grade 3 liver There was a positive correlation between body surface area dysfunction was observed in two patients who had the highest versus clearance of lactone (r2 ϭ 0.11) and total drug (r2 ϭ values of AUClactone and AUCtotal. 0.12), although these correlations were not statistically signifi- Response. Nine patients had stable disease, and six pa- cant (P ϭ 0.24 and 0.20, respectively) because of the small tients had disease progression. No complete or partial responses number of patients. The AUC of the lactone and total drug were observed in this study. increased linearly with dose escalations (Fig. 4); clearance ad- justed by body surface area of lactone (P ϭ 0.39) and total drug (P ϭ 0.50) did not change with dose escalations. Pharmacoki- DISCUSSION netic parameters are listed in Table 4. Concentrations of total Dose-limiting toxicities of exatecan administered over 30 and lactone exatecan declined in parallel during the termination min every 3 weeks were neutropenia and liver dysfunction and phase, and the ratio of the lactone AUC to the total AUC was were observed in two of six patients treated at 6.65 mg/m2; the 0.30. Although UM-1 was detected in all patients, the AUC of recommended dose was 5 mg/m2. Exatecan is being developed UM-1 was 7% or less of the parent drug in plasma. However, with the expectation of enhancing the antitumor activity of the amount of UM-1 excreted in urine in 24 h was twice as much camptothecin derivatives and of reducing the severe toxicity of as that of the parent drug, and 25 Ϯ 8% (mean Ϯ SD) of the irinotecan, especially the unpredictable diarrhea. administered drug was excreted in urine as the parent compound The principal toxicity of exatecan was neutropenia, but it or UM-1. One patient treated at 6.65 mg/m2 had a much higher was easily managed and complicated by fever in only one AUC than the other patients and might seem to be an outlier patient, who had the highest drug exposure. Neutropenia did not (Fig. 4). However, the clearances of lactone and total drug in preclude the planned every-3-week administration in five of six this patient were within the 95% confidential intervals of all patients treated at the recommended dose of 5 mg/m2, and patients. Exatecan is metabolized to UM-1 by CYP3A4, and treatment could be repeated every 4 weeks in the remaining many drugs are known to inhibit the activity of CYP enzymes. patient. The AUC of UM-1 in the patient was also highest among all Dose-limiting liver dysfunction was observed in two pa- patients, and the patient was not taking any drugs known to tients treated at 6.65 mg/m2 in this study. However, only six inhibit CYP3A4. patients were treated at this dose, and the confidence interval for Neutropenia was the principal toxicity, and relationships the rate of the toxicity was large. Furthermore, it was asymp-

between neutropenia versus the AUC of lactone (AUClactone) tomatic and might not be dose-limiting, unless blood chemistries

are measured 2 or 3 days after the treatment. In this study, blood 4.5 4.6 2.1 1.2 chemistries were measured within a week after the first admin- Ϯ Ϯ Ϯ Ϯ

(%) istration of exatecan in 14 of 15 patients, and liver dysfunction UM-1 was recorded in three patients. However, liver dysfunction was transient and normalized in the second week. Therefore, intrapa- 4.2 16.0 5.3 19.2 3.0 15.6 1.6 10.5 tient dose escalation with careful monitoring might be option- Ϯ Ϯ Ϯ Ϯ (%) Urinary excretion ally performed in Phase II studies. A notable feature of the Exatecan toxicity profile of exatecan was that diarrhea was infrequent and mild. In marked contrast to irinotecan, severe diarrhea was not 0.01 8.7 0.01 11.0 0.01 7.6 0.01 6.3

total observed in this study. Diarrhea is dose limiting and sometimes UM-1 Ϯ Ϯ Ϯ Ϯ fatal in treatment with irinotecan, and extensive use of loper- AUC AUC amide is necessary when diarrhea begins after irinotecan administration (25–27). This did not happen in this study of exatecan, 0.08 0.04 0.09 0.04 0.05 0.05 0.12 0.04 and only one episode of grade 2 diarrhea was observed at 5 total

lactone 2 Ϯ Ϯ Ϯ Ϯ mg/m . AUC

AUC The number of patients in this study was small, and the confidence interval for toxicity rates was large. However, an- ) 8 0.30 10 0.28 7 0.33 7 0.29 2 other Phase I study of exatecan, administered using the same dss Ϯ Ϯ Ϯ Ϯ V schedule as our study, has been conducted in France, and similar (l/m results were observed (28). Doses ranging from 4 to 7.1 mg/m2 )

2 were evaluated in the French study, and the recommended dose 1.1 22 0.9 20 1.2 25 0.9 20 SD) Ϯ Ϯ Ϯ Ϯ Cl and toxicities were similar to those of our study. The recom- Ϯ 2 2 (l/h/m mended dose was 5.3 mg/m in the French study (5 mg/m in

b our study), and neutropenia was dose limiting. Similar to our carboxylate) 1/2 (h) study, grade 3 or greater diarrhea was not observed in their T ϩ study. Liver dysfunction was not observed in their study, but as mentioned previously, this might be related to the timing and 6922 12.2 1.6 965 10.2 2.8 812 10.2 1.9 h/ml) frequency of the measurements of blood chemistry. In a Phase Ϯ Ϯ Ϯ ϫ AUC I study of exatecan using a daily-times-five schedule, liver

(ng dysfunction was also observed (29). Although the concentration of the lactone form was not measured in the French study, the 160 7004 147 2092 40 1836 clearance and the half-life of the total drug was 2.4 l/h/m2 and max Ϯ Ϯ Ϯ

C 7.7 h, respectively, and close to values obtained in our study (2.1 (ng/ml) l/h/m2 and 10.9 h). These observations suggest that there are no Pharmacokinetic profile of exatecan (mean

) large interethnic differences in the pharmacokinetics or pharma- 12 10.9 2.1 13 779 11 565 13 320 2 dss , distribution volume at the steady state. Ϯ Ϯ Ϯ Ϯ codynamics of exatecan. However, the numbers of patients V (l/m dss

V treated in both studies were small, and a large population should

Table 4 be evaluated before making definitive conclusions on intereth- ) 2 2.8 38 2.3 36 3.2 36 0.8 45 nic differences. Ϯ Ϯ Ϯ Ϯ Cl Unlike irinotecan, exatecan does not require metabolic (l/h/m activation, which may be beneficial in reducing the interpatient Cl , clearance; b variability of pharmacokinetics of the active compounds. In a 1/2 (h) T Phase I study of irinotecan administered every 3 weeks, the

Lactone Total (lactone coefficient of variation of the lactone AUC of SN-38 was large, , half life; 1058 10.7 5.3 240 8.4 8.5 60 10.3 6.5 h/ml)

1/2 ranging from 63 to 105% when therapeutic doses of 240–345 T Ϯ Ϯ Ϯ 2 ϫ AUC mg/m of irinotecan were administered (30). Similarly, the

(ng coefficient of variation of the lactone AUC of SN-38 was 60–106% in children receiving a 1-h infusion of irinotecan (31). a 149 1591 137 663 43 464 In a pharmacological study of irinotecan at a weekly dose of 100 2 Ϯ Ϯ Ϯ max mg/m , the coefficient of variation was reported to be 34% (32). C (ng/ml) These figures were larger than or similar to the moderate coefficient of variation of the AUC of lactone exatecan in this study (36% for the recommended dose; 5 mg/m2). Similarly, the

No. of variability of the lactone AUC of exatecan seemed to be less patients , the maximum concentration; than that of topotecan administered over 30 min (51%; Ref. 33). ) max 2

C Harmonic mean. The second peak in the concentration-time curve was observed a b 6.65 6 664 5 6 440 3 3 258 Dose Total 15in 10 of 15 9.6 patients 6.8 in this study, which might suggest entero- (mg/m hepatic recirculation. Rebound concentrations were also ob-

Fig. 5 Relationship between neutropenia versus the AUC of lactone

(AUClactone) and the total drug (AUCtotal). Neutropenia expressed as the surviving fraction of neutrophils was signif- 2 ϭ icantly related to the AUClactone (r ϭ 2 ϭ 0.50, P 0.003) and AUCtotal (r 0.40, P ϭ 0.012) by inhibitory sigmoid

Emax models.

served in other camptothecin derivatives, irinotecan and SN-38 lactone and total drug, respectively), and the number of patients (34, 35). in this study was limited. Therefore, clinical significance of the The activity of camptothecin derivatives depends on a correlation was unclear, and further studies are needed to inves- closed lactone ring structure (36), which rapidly hydrolyzes in tigate whether exatecan can be administered based on body aqueous solution at physiological pH to generate an inactive surface area without increasing variability of drug exposure and ring-opened carboxylate, in a reversible and nonenzymatic fash- toxicities. ion (37, 38). The lactone and carboxylate forms exist at an In conclusion, exatecan was a well-tolerated drug with a equilibrium depending on pH; theoretically, there should be favorable toxicity profile compared with irinotecan. Interpatient minimal interpatient variability in the lactone:carboxylate ratio variability of pharmacokinetics was similar to or smaller than if the equilibrium depends only on pH. If the interpatient vari- other camptothecin derivatives. Although objective complete or ability in the ratio was actually minimal, measurements of total partial responses were not observed in this study, exatecan concentration may be sufficient for future pharmacological in- showed greater antitumor activity than other camptothecin de- vestigations. However, interpatient variability of the ratio of the rivatives in preclinical studies and antitumor responses have lactone AUC to the total AUC of exatecan had a coefficient of been observed in clinical Phase I studies of exatecan using other variation of 27%. This variability was similar to that of SN-38 administration schedules. Further clinical studies are warranted. 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Hironobu Minami, Hirofumi Fujii, Tadahiko Igarashi, et al.

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