Modification of the hydroxy lactone ring of camptothecin: inhibition of mammalian topoisomerase I and biological activity

Effects of CKD-602, a new camptothecin anticancer agent, on pregnant does and embryo-fetal development in rabbits

CKD-602 is a newly developed camptothecin anticancer agent. Preclinical studies suggest that it may have greater antitumor activity and lower toxicity than other camptothecin anticancer agents. The potential of CKD-602 to induce developmental toxicity was investigated in the New Zealand White rabbit. Seventy-two artificially inseminated females (artificial insemination=day 0) were distributed among three treatment groups and a control group. CKD-602 was at dose levels of 0, 0.024, 0.048, or 0.096 mg x kg(-1) x day(-1) administered intravenously to pregnant does from days 6 to 18 of gestation. All does were subjected to caesarean section on day 28 of gestation. At 0.096 mg x kg(-1) x day(-1), 2 cases of abortion and 3 cases of death in pregnant rabbits were found in late gestation. In addition, an increase in the embryonic resorptions and a decrease in the litter size were found. At 0.048 mg x kg(-1) x day(-1), a single doe aborted on gestational day 26. An increase in the embryonic resorptions and fetal morphological alterations and a decrease in the litter size were also found. There were no signs of maternal toxicity or developmental toxicity at 0.024 mg x kg(-1) x day(-1). The results show that 13-day repeated intravenous dose of CKD-602 during the major organogenetic period in rabbits produces increased incidence of abortion and death, increased number of embryonic resorptions and fetal morphological alterations, and decreased litter size at dose levels of above 0.048 mg x kg(-1) x day(-1). In the current experimental conditions, the no-observed-adverse-effect levels (NOAELs) of CKD-602 are considered to be 0.048 mg x kg(-1) x day(-1) for does and 0.024 mg x kg(-1) x day(-1) for embryo-fetal development.

Alternative administration of camptothecin analogues

In order to improve the therapeutic index of camptothecin (CPT) analogues, alternative administration of CPT analogues is being evaluated. Topotecan, irinotecan, rubitecan, lurtotecan and 9-aminocamptothecin have been administered orally with response rates equivalent to that seen after intravenous administration, where applicable. Oral availability and administration of some of the newer CPT analogues, including diflomotecan (BN80915) and grimatecan (ST1481), have also shown promising results. Aerosolisation of liposomal 9-nitrocamptothecin has been studied in patients with advanced malignancies involving the lung, demonstrating systemic antitumour activity. Intrathecal administration of topotecan has been studied in children with refractory neoplastic meningitis. It is well tolerated and associated with some antitumour activity. Intraperitoneal administration of topotecan as consolidation therapy in patients with ovarian cancer has shown promising results. Transdermal administration of rubitecan has been studied in mice. So far, no CPT has been approved for an alternative route of administration.

Embryotoxic effects of CKD-602, a new camptothecin anticancer agent, in rats

CKD-602 is a newly developed camptothecin anticancer agent. Preclinical studies suggest that it may have greater antitumor activity and lower toxicity than other camptothecin anticancer agents. The potential of CKD-602 to induce embryotoxicity was investigated in the Sprague-Dawley rat. One hundred mated females (sperm in vaginal lavage=day 0) were distributed among three treatment groups and a control group. CKD-602 was administered intravenously at dose levels of 0, 5, 20 and 80 microg/kg/d to pregnant rats from days 6 to 15 of gestation. The vehicle control rats received an equivalent volume of 1 ml distilled water with d-mannitol 50mg and tartaric acid 0.06 mg. All dams were subjected to the caesarean section on day 20 of gestation. There were no signs of maternal toxicity or embryotoxicity at 5 microg/kg/d, but at 20 microg/kg/d, there was an increase in relative brain weight. At 80 microg/kg/d, reduced food intake, suppressed body weight and increased weight of spleen were observed in dams. An increase in the resorptions and dead fetuses, a decrease in litter size, fetal and placental weights were also found. In addition, various types of external, visceral and skeletal malformations occurred. Characteristic malformations included absent eye bulge, agnathia, dilated cerebral ventricle, anophthalmia, absent thoracic centrum, fused vertebral arch, fused rib, among others. Visceral and skeletal variations were observed. Retarded ossification of several skeletal districts and delayed ossification of sternebrae, metatarsals and sacrocaudal vertebrae were also observed. The results show that CKD-602 is embryotoxic and teratogenic at a minimally maternally toxic dose, i.e. at 80 microg/kg/d in rats. The no-observed-adverse-effect level (NOAEL) of CKD-602 for developmental toxicity was considered to be 20 microg/kg/d, however, the NOAEL for maternal toxicity was 5 microg/kg/d.

A novel non-toxic camptothecin formulation for cancer chemotherapy

The use of a novel injectable biocompatible and biodegradable camptothecin-polymer implant for sustained intra-tumoral release of high concentrations of camptothecin is described. The drug delivery vehicle is an in situ thermogelling formulation, which is based on the natural biopolymer chitosan. This formulation, containing homogeneously dispersed camptothecin, was implanted intra-tumorally into a sub-cutaneous mouse tumor model (RIF-l). The effectiveness of treatment was measured in terms of tumor growth delay (TGD). Animals treated with the polymer implants containing camptothecin had significantly longer TGDs compared to untreated animals as well as to animals treated systemically with camptothecin by intra-peritoneal injection with no evidence of toxicity in terms of loss of body weight. The results indicate that this novel biodegradable polymer implant is an effective vehicle for the sustained intra-tumoral delivery of camptothecin which might also be suitable to deliver other insoluble anti-cancer drugs such as taxol.

Increased susceptibility of spinal muscular atrophy fibroblasts to camptothecin-induced cell death

Spinal muscular atrophy (SMA) is a neuromuscular disease caused by deletions or mutations in the telomeric copy of the survival motor neuron (SMN1) gene. Although the SMN protein has been implicated in the biogenesis of ribonucleoprotein complexes and RNA processing, it is not clear how these functions contribute to the pathogenesis of SMA. To gain a further understanding of SMN function, we have investigated its role in cell survival in skin fibroblasts derived from SMA patients and age-matched controls. SMA fibroblasts exposed to camptothecin, a specific inhibitor of DNA topoisomerase I, consistently showed cell death at a lower concentration than normal controls. Treatment with other cell death-inducing agents did not cause differences in survival of SMA fibroblasts as compared with control fibroblasts. Camptothecin treatment resulted in activation of caspase-3 with generation of the caspase-3 cleavage product, poly ADP-ribose polymerase (PARP). Depletion of SMN protein by RNA interference in control fibroblasts increased caspase-3 activity, whereas transfection of SMA fibroblasts with wild-type SMN decreased caspase-3 activity. Our data demonstrate that SMA fibroblasts are more prone to some, but not all, death-stimuli. Vulnerability to death-stimuli is associated with decreased levels of SMN protein and is mediated by activation of caspase-3.

4-Week repeated intravenous dose toxicity study of a new camptothecin anticancer agent CKD-602 in dogs

CKD-602 is a new camptothecin derivative antitumor agent with a formula (7-[2-(N-isopropylamino)ethyl]-(20S)-camptothecin) developed by Chong Kun Dang Pharmaceutical Company in Korea. In the present study, the subacute toxicity of CKD-602 was investigated after 4-week repeated intravenous administration of the test chemical in beagle dogs. The test chemical was administered intravenously at dose levels of 0, 0.001, 0.005, or 0.01 mg/kg/day for 4 weeks to male and female dogs (n = 3 for male and female dogs for each dose). During the test period, clinical signs, mortality, body weights, food and water consumption, ophthalmoscopy, urinalysis, hematology, serum biochemistry, gross findings, organ weights and histopathology were examined. In the high dose group, an increase in the incidence of abnormal clinical signs and a decrease in food and water intake and body weight gain were observed in both sexes. Hematological investigations revealed decreased white blood cells (WBC) in both sexes and reduced red blood cells (RBC), hemoglobin and hematocrit in females. Histopathological examinations revealed an increase in the incidence of atrophy of the sternal and femoral marrow and spleen in both sexes and atrophy of the thymus and mesenteric lymph node in males. No treatment-related adverse effects were observed in both sexes of the low and middle dose groups. In conclusion, the 4-week repeated intravenous dose of CKD-602 to beagle dogs caused increases in the clinical signs and histopathological changes, and decreases in the body weight gain, food and water intake, RBC, hemoglobin, hematocrit and WBC at the dose level of 0.01 mg/kg/day. In the present experimental conditions, the target organs were determined to be bone marrow, blood cells, spleen, thymus, and mesenteric lymph node. The no-observed-adverse-effect levels (NOAEL) for males and females were considered to be 0.005 mg/kg/day, respectively.

Prevention of Irinotecan-Induced Diarrhea by Oral Sodium Bicarbonate and Influence on Pharmacokinetics

Alkalization of the intestinal tract by oral administration of sodium bicarbonate has been reported to be a promising method for preventing delayed diarrhea, a dose-limiting toxicity in patients receiving chemotherapy with irinotecan hydrochloride. However, it is feared that this method may adversely affect the pharmacokinetics of irinotecan by inhibiting its intestinal absorption and that of its active metabolites. We compared the pharmacokinetics and toxicity of irinotecan with and without oral alkalization in a cross-over study that enrolled 10 colorectal cancer patients. We found that alkalization did not decrease the blood levels of irinotecan and its active metabolite. In fact, the area under concentration versus time curves (AUCs) of irinotecan and 7-ethyl-10-hydroxycamptothecin glucuronide (SN-38G) were statistically equivalent both with and without oral alkalization. Also, the AUC of SN-38 with alkalization was statistically equivalent or larger than that without alkalization. Oral alkalization reduced the incidence of diarrhea and gastrointestinal symptoms, and these adverse effects were not worsened by long-term administration. These results suggest that oral alkalization can control diarrhea and gastrointestinal toxicity without decreasing the blood levels of irinotecan and its active metabolites, thus improving the tolerability of long-term chemotherapy without reducing efficacy.

Synthesis of 14-Azacamptothecin, a Water-Soluble Topoisomerase I Poison

14-Azacamptothecin, a potent, water-soluble analogue of the antitumor agent camptothecin, has been prepared by a convergent synthesis. The key condensation of the AB and DE rings with concomitant formation of ring C of 14-aza CPT was carried out in two stages, the latter of which involved a radical cyclization strategy. [structure: see text]

On the role of E-ring oxygen atoms in the binding of camptothecin to the topoisomerase I–DNA covalent binary complex

A recent X-ray crystallographic analysis of the binding of a water soluble camptothecin analogue to the human topoisomerase I-DNA covalent binary complex has suggested the existence of some novel features in the way that camptothecin is bound to the binary complex. Four additional models based on chemical and biochemical data have also been proposed. Presently we describe S-containing analogues of camptothecin prepared on the basis of these models, and report their ability to form stable ternary complexes with human topoisomerase I, and to mediate cytotoxicity at the locus of topoisomerase I. The results indicate that replacement of the 20-OH group of CPT with a SH functionality results in diminution of the potency of CPT as a topoisomerase I poison, while replacement of the O atoms at positions 20 and 21 with S atoms results in essentially complete loss of topoisomerase I inhibitory activity.

14-Azacamptothecin: A Potent Water-Soluble Topoisomerase I Poison

On the basis of an analysis of luotonin A and its D-ring deaza analogue as topoisomerase I poisons and topoisomerase I-dependent cytotoxic agents, a novel analogue of the structurally related antitumor antibiotic camptothecin (CPT) was prepared. 14-Azacamptothecin was found to have much greater aqueous solubility than CPT, to inhibit topoisomerase I-mediated DNA relaxation more efficiently than CPT, and to stabilize the covalent binary complex to almost the same extent. 14-Aza CPT was found to be slightly less active than CPT in mediating cytotoxicity toward yeast expressing human topoisomerase I, possibly as a consequence of its greater off-rate from the CPT-topoisomerase I-DNA ternary complex.

Subacute toxicity evaluation of a new camptothecin anticancer agent CKD-602 administered by intravenous injection to rats

The subacute toxicity of a new camptothecin anticancer agent, CKD-602, was investigated after 4-week repeated intravenous administration of the chemical in Sprague-Dawley rats. The test chemical was administered intravenously to rats at dose levels of 0, 0.003, 0.013, or 0.067 mg/kg/day for males and 0, 0.004, 0.018, or 0.089 mg/kg/day for females. At the end of the treatment period, 10 rats/sex/group were sacrificed. The remaining 5 rats/sex in the vehicle control and high dose groups continued the study without treatment for 2 weeks (recovery period). During the test period, clinical signs, mortality, body weights, food and water consumption, ophthalmoscopy, urinalysis, hematology, serum biochemistry, gross findings, organ weights, and histopathology were examined. In both sexes of the high dose group, an increase in the incidence of abnormal clinical signs and paleness of the eyes, a reduction in the body weight gain, food consumption and urine protein, and an increase in the water consumption were observed. Hematological investigations revealed a decrease in the red blood cells, hemoglobin and hematocrit and an increase in the mean corpuscular volume, mean corpuscular hemoglobin, platelets, and reticulocytes in a dose-dependent manner. Serum total cholesterol and total protein values were lower in females than those of controls, but not in males. An increase in the heart and liver weights and a decrease in the thymus weight were also found. Histopathological alterations included an increase in the incidence of atrophy of the sternal marrow, atrophy, fibrosis and mast cell hyperplasia of the femoral marrow, atrophy of the white pulp and extramedullary hematopoiesis of the spleen, atrophy of the thymus, auricular hypertrophy of the heart, extramedullary hematopoiesis and centriacinar telangiectasis of the liver, follicular degeneration of the ovary, and inflammation of the tail. The major treatment-related effects were not recovered at the end of 2-week recovery period. There were no adverse effects in the low and middle dose groups of both genders. In the present experimental conditions, the target organs were determined to be bone marrow, blood cells, spleen, liver, thymus, and heart. The no-observed-adverse-effect level was considered to be 0.013 mg/kg/day for males and 0.018 mg/kg/day for females.

Camptothecin: current perspectives

This review provides a detailed discussion of recent advances in the medicinal chemistry of camptothecin, a potent antitumor antibiotic. Two camptothecin analogues are presently approved for use in the clinic as antitumor agents and several others are in clinical trials. Camptothecin possesses a novel mechanism of action involving the inhibition of DNA relaxation by DNA topoisomerase I, and more specifically the stabilization of a covalent binary complex formed between topoisomerase I and DNA. This review summarizes the current status of studies of the mechanism of action of camptothecin, including topoisomerase I inhibition and additional cellular responses. Modern synthetic approaches to camptothecin and several of the semi-synthetic methods are also discussed. Finally, a systematic evaluation of novel and important analogues of camptothecin and their contribution to the current structure-activity profile are considered.

Isolated hepatic perfusion for the treatment of colorectal metastases confined to the liver: recent trends and perspectives

Isolated hepatic perfusion (IHP) involves a method of complete vascular isolation of the liver to allow treatment of liver tumours with toxic systemic doses. The recent clinical studies mainly employed IHP with melphalan with or without tumour necrosis factor-alpha (TNF-alpha) and mild hyperthermia. The results of these studies show that high response rates and high survival rates can be achieved by IHP. In this article, the current status, recent developments and future perspectives of IHP are discussed.

Cytotoxic effect of different camptothecin formulations on human colon carcinoma in vitro

Two innovative 20-S-camptothecin (CPT) formulations, previously found suitable to achieve therapeutically relevant CPT concentrations, were assessed for their in vitro cytotoxic potential as compared to an aqueous CPT solution, using the MTT assay. The formulations, cationic CPT-containing liposomes (CPT-Lip), hydroxypropyl-beta-cyclodextrin (HP-beta-CD) complexed CPT (CPT-CD) and a saturated aqueous CPT solution (CPT-Sol), were diluted in culture medium to appropriate CPT concentrations (4.7-300 ng/ml), and incubated with HT-29 and SW-480 human colon carcinoma cell lines. IC50 values were calculated after 48 and 72 h incubation for the HT-29 and SW-480 cell lines, respectively, and were found to be of the same magnitude for all formulations, with only a slight difference (CPT-Sol<CPT-CD<CPT-lip). The cells obtained apoptotic morphology after 36 h incubation with CPT-CD and were demonstrated to be active caspase-3 immuno-positive. Both formulations investigated, CPT-CD and CPT-Lip, showed significant cytotoxicity in vitro relative to CPT-Sol and warrant investigation for future therapeutic application.

Phase II study of irinotecan combined with carboplatin in previously untreated non-small-cell lung cancer

PURPOSE

Irinotecan, a topoisomerase I inhibitor, is an effective agent for non-small-cell lung cancer (NSCLC). To determine the efficacy and toxicity of irinotecan and carboplatin, we conducted a phase II study in 61 patients with advanced NSCLC.

METHODS

Every 4 weeks, the patients received irinotecan 50 mg/m2 (days 1, 8 and 15) and carboplatin (day 1) with a target AUC of 5 mg min/ml using the Chatelut formula.

RESULTS

All patients were evaluable for toxicity, and of 59 patients evaluable for response, 20 achieved a partial response and 26 showed no change. The overall response rate was 34% (95% confidence interval 23-48%). Grade 3 or 4 anemia, leukopenia, neutropenia, thrombocytopenia and diarrhea occurred in 32%, 32%, 60%, 25%, and 7%, respectively. The median survival time and 1-year, and 2-year survival rates were 10.0 months, 37.6%, and 15.2%, respectively.

CONCLUSIONS

Irinotecan with carboplatin is effective for advanced NSCLC and safe.

9-Nitrocamptothecin as second line chemotherapy for men with progressive, metastatic, hormone refractory prostate cancer: Results of the CALGB 99901

BACKGROUND

Institution of early hormone therapy in the PSA era coupled with demonstration of clinical benefit with chemotherapy in hormone refractory prostate cancer (HRPC) and acceptance of PSA decline as a surrogate for response has resulted in introduction of chemotherapy earlier in the natural history of disease. There now exists a need to identify, effective agents for second line chemotherapy. 9-nitrocamptothecin (9-NC) a novel, oral camptothecin analogue was tested as second line chemotherapy for patients with progressive hormone refractory prostate cancer.

PATIENTS AND METHODS

Eligible patients had metastatic hormone refractory prostate cancer with performance status (0-1) following progression on at least 1 prior cytotoxic chemotherapy. 9-NC was administered orally at the dose of 1.5 mg/m2/d for 5 days each week for 3 weeks, followed by rest for 1 week. Response was evaluated after 2 cycles according to the guidelines set forth for Phase II trials in HRPC by the PSA working group.

RESULTS

Thirty-five patients were recruited to the study within a period of 6 months; 33 were evaluable for analysis. No patients had a >50% decline in PSA levels. Two out of 8 (25%) patients with measurable disease and 5/25 (20%) patients with nonmeasurable disease showed stable disease. The median time to disease and PSA progression was 2 months [95% confidence interval (CI), 0.9-2.8]. The median overall survival was 10 months (95% CI = 5-12). Seven patients are alive after a median follow-up of 23 months.

CONCLUSIONS

9-nitrocamptothecin failed to elicit clinical or PSA responses. Further study in pretreated HRPC patients is not warranted.

Degradation of camptothecin-20(S)-glycinate ester prodrug under physiological conditions

We have compared the strikingly different decomposition pathways for camptothecin-20(S)-acetate -acetate and camptothecin-20(S)-glycinate in phosphate buffered saline, human plasma and blood. The aliphatic ester analog camptothecin-20(S)-acetate demonstrated excellent stability in the above fluids for many hours with minimal hydrolysis, while the camptothecin-20(S)-glycinate analog (differing solely by the presence of an amino group) underwent rapid and essentially complete decomposition. Reversed-phase high performance liquid chromatography (RP-HPLC) with electrospray ionization-mass spectral (ESI-MS) detection was then used to correlate structural information for camptothecin-20(S)-glycinate decomposition products. ESI-MS detection indicated the ring-opened carboxylate form of camptothecin and the ring-opened degradation product co-elute near the solvent front, while the latest eluting decomposition product was the closed-ring lactone form of camptothecin. A novel decomposition product with intermediate retention time displayed an identical mass-to-charge ratio as camptothecin-20(S)-glycinate ester but a strikingly different fragmentation pattern. The LC-ESI-MS evidence of a novel camptothecin prodrug degradation pathway is provided in this report.

A method to determine the incorporation capacity of camptothecin in liposomes

The purpose of this study was to establish a new experimental approach to determine the maximum amount of camptothecin (CPT) that can be incorporated in liposomes, and to use this method to compare the CPT-incorporation capacity of various liposome formulations. Small, CPT-saturated liposomes were prepared by dispersing freeze-dried blends of lipids and drug in phosphate buffer, and subsequent probe-sonication. Excess precipitated CPT could be separated from the liposomes by ultracentrifugation. The small and homogeneous liposome size obtained gave a good and reproducible recovery of liposomes in the supernatant (>80%), whereas the acidic pH (pH 6.0) kept CPT in its hydrophobic lactone form, which is poorly soluble in the buffer. The maximum CPT-incorporation capacity of 12 different liposome formulations was investigated, using the described method, and was found to vary widely. With liposomes made of neutral and anionic phospholipids, the solubility of CPT in the buffer was improved by approximately a factor of 10 (from 2.7 to 15-50 microg/mL) as compared with buffer. With cationic liposomes containing 1,2-dioleoyl-3-trimethyl-ammonium-propane (DOTAP), a maximum CPT-solubilization of 100-fold, the buffer solubility was reached, probably owing to an electrostatic interaction between the cationic lipids and the carboxylate-CPT isomer. Increasing DOTAP fractions within egg-phosphatidylcholine (EPC)/DOTAP liposomes reached a CPT-incorporation plateau at 20 mol% DOTAP. The presented approach appears suitable to study the incorporation capacity of any drug component within small vesicles as long as the liposome incorporation is high relative to the intrinsic water solubility of the drug.

Determination of irinotecan (CPT-11) and SN-38 in human whole blood and red blood cells by liquid chromatography with fluorescence detection

An analytical method was developed for the anticancer agent irinotecan (CPT-11) and its main metabolite SN-38 in human whole blood and in red blood cells (RBCs). Sample pretreatment involved deproteinization of whole blood or plasma-diluted RBCs isolated by MESED instruments, with a mixture of aqueous perchloric acid and methanol (1:1, v/v). Separation was carried out using isocratic elution on a Hypersil ODS stationary phase, with detection at excitation and emission wavelengths of 355 and 515 nm, respectively. The lower limit of quantitation (LLQ) in blood was established at 5.00 ng/ml for both compounds, with values for within-run precision (WRP) and between-run precision (BRP) of less than 10%. The method is currently being applied to investigate the blood distribution of CPT-11 and SN-38 in cancer patients.

Membrane transport of camptothecin: facilitation by human P-glycoprotein (ABCB1) and multidrug resistance protein 2 (ABCC2)

BACKGROUND

The purpose of the present study was to continue the investigation of the membrane transport mechanisms of 20-(S)-camptothecin (CPT) in order to understand the possible role of membrane transporters on its oral bioavailability and disposition.

METHODS

The intestinal transport kinetics of CPT were characterized using Caco-2 cells, MDCKII wild-type cells and MDCKII cells transfected with human P-glycoprotein (PGP) (ABCB1) or human multidrug resistance protein 2 (MRP2) (ABCC2). The effects of drug concentration, inhibitors and temperature on CPT directional permeability were determined.

RESULTS

The absorptive (apical to basolateral) and secretory (basolateral to apical) permeabilities of CPT were found to be saturable. Reduced secretory CPT permeabilities with decreasing temperatures suggests the involvement of an active, transporter-mediated secretory pathway. In the presence of etoposide, the CPT secretory permeability decreased 25.6%. However, inhibition was greater in the presence of PGP and of the breast cancer resistant protein inhibitor, GF120918 (52.5%). The involvement of additional secretory transporters was suggested since the basolateral to apical permeability of CPT was not further reduced in the presence of increasing concentrations of GF120918. To investigate the involvement of specific apically-located secretory membrane transporters, CPT transport studies were conducted using MDCKII/PGP cells and MDCKII/MRP2 cells. CPT carrier-mediated permeability was approximately twofold greater in MDCKII/PGP cells and MDCKII/MRP2 cells than in MDCKII/wild-type cells, while the apparent Km values were comparable in all three cell lines. The efflux ratio of CPT in MDCKII/PGP in the presence of 0.2 microM GF120918 was not completely reversed (3.36 to 1.49). However, the decrease in the efflux ratio of CPT in MDCKII/MRP2 cells (2.31 to 1.03) suggests that CPT efflux was completely inhibited by MK571, a potent inhibitor of the Multidrug Resistance Protein transporter family.

CONCLUSIONS

The current results provide evidence that PGP and MRP2 mediate the secretory transport of CPT in vitro. However, the involvement of other transporters cannot be ruled out based on these studies. Since these transporters are expressed in the intestine, liver and kidney variations in their expression levels and/or regulation may be responsible for the erratic oral absorption and biliary excretion of CPT observed in human subjects.

Transport characteristics of 9-nitrocamptothecin in the human intestinal cell line Caco-2 and everted gut sacs

The intestinal absorptive characteristics and the efflux mechanisms of 9-nitrocamptothecin (9-NC), a novel water-insoluble camptothecin (CPT) derivative, were investigated. The Caco-2 cells and the everted gut sacs were used as models of the intestinal mucosa to assess transepithelial transport of 9-NC. The determination of 9-NC was performed by HPLC. In the Caco-2 cells, the absorptive transport of 9-NC was pH dependent and the transport was enhanced at weakly acidic pH on the apical side. No concentration dependence and saturation were observed for the absorptive transport of 9-NC at concentrations up to 250 microM, while secretory transport were concentration dependent and saturable process (K(m) was 49.8 +/- 1.2 microM, V(max) was 38.28 +/- 0.8 ng/cm(2)/min). In the presence of verapamil (100 microM) and CsA (10 microM), potent inhibitors of P-glyprotein (P-gp)/MRP2 (cMOAT), the P(appBL-AP)/P(appAP-BL) ratio was decreased from 3.4 to 1.4 and 1.3, respectively, and permeation of apical to basolateral was enhanced approximately two-fold. In the everted gut sacs, the absorption of 9-NC was passive diffusion and had no significant difference in different gut regions. Adding verapamil in the everted gut sacs over a concentration ranging from 10 to 100 microM, the absorption of 9-NC was significantly enhanced, especially more markedly in lower small intestine (P < 0.05). Overall, the current study suggests that pH and efflux transporters are capable of mediating the absorption and efflux of 9-NC, and they may play significant roles in limiting the oral absorption of 9-NC.

Synthesis of novel 2,3-dihydro-1,4-dioxino[2,3- g ]quinoline derivatives as potential antitumor agents

New dioxinoquinolines (1-8) have been synthesized and their antiproliferative properties have been tested against several cell lines. The treatment of the 6-acetamido-2,3-dihydro-1,4-benzodioxine (10) with phosphorous oxychloride in the presence of DMF leads to a mixture of linear and angular tricyclic compounds. The key intermediates were modified and cyclized giving the corresponding dioxinoquinolines. In general, these compounds have a moderate citotoxycity.

Mechanisms of resistance to topoisomerase I-targeting drugs

DNA topoisomerases are a class of enzymes that alter the topology of DNA and are targets of several anticancer drugs. Camptothecins (CPTs) are a relatively new family of compounds that specifically target topoisomerase I (Top1). These compounds "poison" Top1 by binding to the Top1-DNA complex in a manner that prevents the religation of DNA. Topotecan and irinotecan are two CPTs that are approved for the treatment of a variety of malignancies, including colorectal, ovarian, and small cell lung cancers, as well as myeloid malignancies. Although CPTs have proven to be effective anticancer drugs, resistance is still a critical clinical problem. The mechanisms underlying de novo and acquired clinical resistance to CPTs and the newer classes of Top1 poisons are unclear. However, based on preclinical studies, it is likely that clinical resistance to these drugs is the result of: (1) inadequate accumulation of drug in the tumor, (2) resistance-conferring alterations in Top1, or (3) alterations in the cellular response to the Top1-CPT interaction. This review will focus on the current knowledge regarding mechanisms of resistance to CPTs and other Top1-targeting drugs.

Luotonin A. A Naturally Occurring Human DNA Topoisomerase I Poison

Luotonin A is a pyrroloquinazolinoquinoline alkaloid isolated from the Chinese herbal medicinal plant Peganum nigellastrum. Although previously shown to exhibit cytotoxicity against the murine leukemia P-388 cell line, the mechanism of action of luotonin A is unknown. Presently, we demonstrate that luotonin A stabilizes the human DNA topoisomerase I-DNA covalent binary complex, affording the same pattern of cleavage as the structurally related topoisomerase I inhibitor camptothecin. Luotonin A also mediated topoisomerase I-dependent cytotoxicity toward Saccharyomyces cerevisiae lacking yeast topoisomerase I, but harboring a plasmid having the human topoisomerase I gene under the control of a galactose promoter. This finding identifies a putative biochemical locus for the cytotoxic action of luotonin A and has important implications for the mechanism of action of camptothecin and the design of camptothecin analogues.

Design, synthesis, and biological evaluation of cytotoxic 11-alkenylindenoisoquinoline topoisomerase I inhibitors and indenoisoquinoline-camptothecin hybrids

The indenoisoquinolines are a novel class of topoisomerase I (top1) inhibitors that are cytotoxic in cancer cell cultures and are therefore under development as potential anticancer agents. As inhibitors of the DNA religation reaction occurring after DNA cleavage by the enzyme, they are classified as top1 poisons, similar to the camptothecins. Two strategies were employed in order to further develop the structure-activity relationships of the indenoisoquinolines and enhance their therapeutic potential. The first strategy involved the synthesis of indenoisoquinoline-camptothecin hybrid molecules to take advantage of a proposed structural analogy between the indenoisoquinolines and camptothecin. The desired hybrids were synthesized by reaction of halogenated phthalides with a dihydropyrroloquinoline. The second strategy involved the attachment of various alkenyl substituents to the C-11 position of the indenoisoquinolines, which were assumed to project into the DNA minor groove. The required C-11-substituted indenoisoquinolines were synthesized by McMurry reactions of 11-ketoindenoisoquinolines with aldehydes, and the geometries of the resulting alkenes were established by nuclear Overhauser effect difference NMR spectroscopy. All of the new indenoisoquinolines were examined for cytotoxicity in human cancer cell cultures as well as for activity vs top1. Although the indenoisoquinoline-camptothecin hybrid molecules proved to be less cytotoxic and displayed less activity against top1, an analogue incorporating a 3'-aminoalkenyl substituent at the C-11 position of the indenoisoquinoline system was significantly more potent than the prototype indenoisoquinoline in both assays. These results indicate that C-11 aminoalkyl substituents that are assumed to project into the minor groove enhance the cytotoxicity and top1 inhibitory activity of the parent indenoisoquinoline system.

A mechanistic and kinetic study of the E-ring hydrolysis and lactonization of a novel phosphoryloxymethyl prodrug of camptothecin

PURPOSE

This study was done to determine the E-ring hydrolysis and lactonization mechanism of a water-soluble 20-phosphoryloxymethyl (POM) prodrug of camptothecin (P-CPT). Specifically, the role of the phosphate group in facilitating E-ring hydrolysis was examined.

METHODS

Resolution between the lactone and carboxylate forms of P-CPT and camptothecin (CPT) was achieved with a RPHPLC assay using UV-visible detection. E-ring P-CPT hydrolysis and lactonization kinetics were followed using 20 mM acetate or phosphate buffer (micro = 0.15 NaCl) over the pH range of 4 to 8 at 25.0 degrees C. A kinetic solvent isotope effect (KSIE) study was used to further probe the mechanism of E-ring hydrolysis.

RESULTS

The hydrolysis and lactonization reactions followed pseudo-first-order kinetics in the approach to equilibrium. The equilibrium ratio of the open and closed forms of P-CPT was dependent on pH, with the closed form dominant at low pH and the open form dominant at high pH. Buffer concentration changes had little to no effect on the rate of P-CPT E-ring hydrolysis. The KSIE study provided an overall isotope effect of 2.47 and a proton inventory KSIE consistent with an intramolecular general base catalysis.

CONCLUSIONS

P-CPT has a pH-dependent equilibrium between the lactone and carboxylate forms similar but not identical to that of CPT. The results suggest a hydrolysis reaction mechanism that involves a single site hydrogen exchange facilitated intramolecularly by the dianionic phosphate moiety of P-CPT via either general base catalysis of the lactone ring attack by water or breakdown of the tetrahedral intermediate.

Some new bi- and ter-benzimidazole derivatives as topoisomerase I inhibitors

The discovery of DNA topoisomerases has added a new dimension to the study of anticancer drugs. In the last years detailed investigation of bi- and ter-benzimidazole derivatives revealed that these compounds are a new class of topoisomerase I inhibitors that poisons mammalian topoisomerase I. In this context a survey about topoisomerase I poisoning activity and cytotoxicity of bi- and ter-benzimidazoles is given. Moreover some recent results about new derivatives, some structure-activity relationships and comparison of activity of various functional groups are discussed.

Validation of a novel assay for checkpoint responses: characterization of camptothecin derivatives in Saccharomyces cerevisiae

The evolutionary conservation of pathways preserving genetic stability supports the use of a lower eukaryote such as the yeast Saccharomyces cerevisiae in screening for novel anti-neoplastic agents. Yeast is already established as a model system to characterize the cellular effects of the topoisomerase inhibitor and anti-cancer agent camptothecin (CPT). Here, we demonstrate that a recently developed two-hybrid based plate assay that visualizes the DNA damage-induced homomeric complex formation of the yeast checkpoint protein Rad17 correctly predicts the biological activity of the tested camptothecin derivatives. The used criteria for biological activity include lethality, cell cycle arrest and Rad53p phosphorylation, an essential signaling event during checkpoint activation. Surprisingly, although responsive to camptothecin and not without influence on drug sensitivity, Rad17p appears to be dispensable for cell cycle arrest and for Rad53p phosphorylation following treatment with camptothecin. Such a role is only uncovered if double-strand break repair is compromised.

Tripartate poly(ethylene glycol) prodrugs of the open lactone form of camptothecin

Two PEG prodrugs utilizing conjugation of PEG through the C-21 acid functionality as well as the C-17 OH group of CPT hydroxy-amide open forms were synthesized and characterized. Both of these open lactone tripartate prodrugs were shown to be water soluble and highly effective in MX-1 mouse xenograph studies. Indirect evidence implies that the initial ester or carbonate bond breaking is esterase mediated in the first step of the cascade of CPT release.

High-performance liquid chromatographic assay with fluorescence detection for the simultaneous measurement of carboxylate and lactone forms of irinotecan and three metabolites in human plasma

Irinotecan (CPT-11), a camptothecin analog, is metabolized to SN-38, an active topoisomerase I inhibitor, and inactive metabolites, including APC and SN-38 glucuronide (SN-38G). A high-performance liquid chromatographic assay method to simultaneously measure the lactone and carboxylate forms of CPT-11, SN-38, SN-38G, and APC in human plasma was developed. Chromatography was accomplished with a reversed-phase C(8) column and fluorescence detection. A gradient mobile phase system was used. The buffer for mobile phase A consisted of 0.75 M ammonium acetate, 5 mM tetrabutylammonium phosphate (pH 6.0), and acetonitrile (86:14, v/v). The buffer for mobile phase B was identical to mobile phase A with the exception of the concentration (50:50, v/v). Precipitation of plasma proteins was performed with cold methanol. The linear range of detection of the lactone and carboxylate forms of SN-38, SN-38G, and APC was 2-25 ng/ml, and 5-300 ng/ml for CPT-11. The limit of quantitation for the analytes ranged from 0.5 to 5 ng/ml. Analysis of patients' plasma samples obtained before and after CPT-11 administration showed that the assay is suitable for measuring lactone and carboxylate forms of CPT-11, SN-38, SN-38G, and APC in clinical studies.

Effect of intrapatient dosage escalation of irinotecan on its pharmacokinetics in pediatric patients who have high-grade gliomas and receive enzyme-inducing anticonvulsant therapy

The authors set out to determine the effect of intrapatient dose escalation of irinotecan on its disposition in pediatric patients with high-grade glioma who received concomitant enzyme-inducing anticonvulsants (EIAs). During Course 1, a 60-minute intravenous infusion of irinotecan (20 mg/m(2) per day) was administered once daily for 5 days on each of 2 consecutive weeks. The authors measured the concentrations of the lactone forms of irinotecan and its metabolites 7-ethyl-10-hydroxycamptothecin (SN-38), SN-38 glucuronide, and 7-ethyl-10-[4-N-(5-aminopeptanoic acid)-1-piperidino]-carbonyloxycamptothecin (APC) in serial plasma samples collected on Days 1 and 12 of Course 1. For the 6 patients who received EIAs but whose SN-38 areas under the concentration-time curve (AUCs) on Day 1 were below clinically significant levels, irinotecan dosage was increased, and subsequent pharmacokinetic studies were performed. Thirty-five patients were enrolled. The rate of irinotecan clearance was greater for patients who received EIAs than for those who did not (P = 0.0008), whereas systemic exposure to irinotecan (P = 0.02) and SN-38 (P = 0.0001) was lower for those treated with EIAs than for those who were not. Of the 6 patients whose irinotecan dosages were increased, 3 experienced an increase in the SN-38 AUC between Days 1 and 12. For 1 patient, the SN-38 AUC on Day 12 was lower than on Day 1; this result likely was due to an increased dose of EIAs during the same period. Despite irinotecan dose escalation to 60 and 80 mg/m(2), the SN-38 AUCs for 2 patients did not increase to clinically significant levels. The type and grade of toxicity did not differ between the two patient groups. Increasing the dosage of irinotecan increased the SN-38 AUC in some patients who received concomitant EIA therapy.

Further studies on 2,4-diamino-5-(2',5'-disubstituted benzyl)pyrimidines as potent and selective inhibitors of dihydrofolate reductases from three major opportunistic pathogens of AIDS

As part of an ongoing effort to discover novel small-molecule antifolates combining the enzyme-binding species selectivity of trimethoprim (TMP) with the potency of piritrexim (PTX), 10 previously unreported 2,4-diamino-5-(2'-methoxy-5'-substituted)benzylpyrimidines (2-11) containing a carboxyl group at the distal end of the 5'-substituent were synthesized and tested as inhibitors of dihydrofolate reductase (DHFR) from Pneumocystis carinii (Pc), Toxoplasma gondii (Tg), and Mycobacterium avium (Ma), three of the opportunistic pathogens frequently responsible for life-threatening illness in people with impaired immune systems as a result of HIV infection or immunosuppressive chemotherapy. The selectivity index of DHFR inhibition was evaluated by comparing the potency of each compound against the parasite enzymes with its potency against rat liver DHFR. 2,4-Diamino-5-[5'-(5-carboxy-1-pentynyl)-2'-methoxybenzyl]pyrimidine (3) inhibited Pc DHFR with a selectivity index of 79 and was 430 times more potent than TMP. 2,4-Diamino-5-[5'-(4-carboxy-1-butynyl)-2'-methoxybenzyl]pyrimidine (2), with one less carbon than 3 in the side chain, had a selectivity index of 910 against Ma DHFR and was 43 times more potent than TMP. 2,4-Diamino-5-[5'-(5-carboxypentyl)-2'-methoxybenzyl]pyrimidine (6) had a selectivity index of 490 against Tg DHFR and was 320 times more potent than TMP. 2,4-Diamino-5-[5'-(6-carboxy-1-hexynyl)-2'-methoxybenzyl]pyrimidine (4), with one more carbon than 3, was less potent against all three of the parasite enzymes than either 3 or 6 and also had a lower selectivity index than 3 against the Pc enzyme. However, 4 was the only member of the series with a selectivity index of >300 against both Tg and Ma DHFR. Given that PTX is at least 10 times more potent against rat DHFR than against P. carinii or T. gondii DHFR and that the selectivity index of several of the compounds matches or exceeds that of TMP as well as PTX, our results suggest that it may be possible to develop clinically useful nonclassical antifolates that are both potent and selective against the major opportunistic pathogens of AIDS.

Stable supersaturated aqueous solutions of silatecan 7-t-butyldimethylsilyl-10-hydroxycamptothecin via chemical conversion in the presence of a chemically modified beta-cyclodextrin

PURPOSE

A method for obtaining clear supersaturated aqueous solutions for parenteral administration of the poorly soluble experimental anti-cancer drug silatecan 7-t-butyldimethylsilyl-10-hydroxycamptothecin (DB-67) has been developed.

METHODS

Equilibrium solubilities of DB-67 were determined in various solvents and pH values, and in the presence of chemically modified water-soluble beta-cyclodextrins. The stoichiometry and binding constants for complexes of the lactone form of DB-67 and its ring-opened carboxylate with sulfobutyl ether and 2-hydroxypropyl substituted beta-cyclodextrins (SBE-CD and HP-CD) were obtained by solubility and circular dichroism spectroscopy, respectively. Kinetics for the reversible ring-opening of DB-67 in aqueous solution and for lactone precipitation were determined by HPLC with UV detection.

RESULTS

Solubilities of DB-67 lactone in various injectable solvent systems were found to be at least one order of magnitude below the target concentration (2 mg/ml). DB-67 forms inclusion complexes with SBE-CD and HP-CD but the solubilization attainable is substantially less than the target concentration. Slow addition of DB-67/ DMSO into 22.2% (w/v) SBE-CD failed to yield stable supersaturated solutions due to precipitation. Stable supersatured solutions were obtained, however, by mixing a concentrated alkaline aqueous solution of DB-67 carboxylate with an acidified 22.2% (w/v) SBE-CD solution. Ring-closure yielded supersaturated solutions that could be lyophilized and reconstituted to clear, stable, supersaturated solutions.

CONCLUSIONS

The method developed provides an alternative to colloidal dispersions (e.g., liposomal suspensions, emulsions, etc.) for parenteral administration of lipophilic camptothecin analogs.

Red blood cells: a neglected compartment in topotecan pharmacokinetic analysis

Previously, a gender dependency of topotecan was found in the pharmacokinetics in the plasma compartment. Here, we prospectively studied the red blood cell (RBC) partitioning of topotecan and evaluated its consequences for overall drug disposition. Blood samples were obtained from 12 patients receiving cisplatin followed by i.v. topotecan. Topotecan pharmacokinetic analysis was performed in whole blood, plasma and RBCs. Significantly slower clearance was noted in females (n=7) compared to males (n=5) for lactone and total topotecan in plasma (p<0.0001), and for total drug in RBCs (p=0.027), but not in whole blood. In addition, no gender-dependent differences were observed in the terminal half-lives of topotecan in any of the compartments. The area under the curve ratios for RBC total to plasma lactone were 2.53+/-0.0640 and 2.13+/-0.442 in males and females, respectively. Hence, topotecan displays preferential affinity for RBCs compared to plasma, although these cells do not act as a depot in which drug accumulates over time. RBCs thus play a principal role in the distribution kinetics of topotecan and have a major impact on its plasma pharmacokinetics. The data warrant a change from current practice in pharmacokinetic studies with this agent and provide further evidence that, in general, the choice of the appropriate assay matrix should be rationally based.

Synthesis and in vivo antitumor activity of poly(l-glutamic acid) conjugates of 20S-camptothecin

Poly-alpha-(l-glutamic acid) (PG) conjugates of 20(S)-camptothecin (1, CPT) displayed improved aqueous solubility compared to CPT, were stable in aqueous solution at neutral pH, and were potent antitumor agents in vivo. Evaluation of PG molecular weight, CPT loading, aqueous solubility, and CPT equivalent dosing with respect to in vivo antitumor potencies of various linked conjugates led to identification of a preferred conjugate composition.

Homocamptothecins: potent topoisomerase I inhibitors and promising anticancer drugs

Homocamptothecins (hCPTs) represent a new generation of antitumor agents targeting DNA topoisomerase I. The expanded seven-membered lactone E-ring that characterizes hCPTs enhances the plasma stability of the drug and reinforces the inhibition of topoisomerase I compared with conventional six-membered CPTs. hCPTs are more efficient than the CPTs at promoting cleavage at T/G sites and induce additional cleavage at C/G sites. Compound BN80765 and its difluoro analogue diflomotecan (DN80915) are potent cytotoxic agents and efficiently induce apoptosis in tumor cells. They display strong antiproliferative activities against specific tumor types. Diflomotecan is remarkably efficient at inhibiting the growth of human colon cancer cells in vivo and, administered orally, it also shows superior activities against human prostate cancers compared with the benchmark products topotecan (TPT) and irinotecan (IRT). Diflomotecan has entered phase I clinical testing and antitumor activity has been observed in patients. This 9,10-difluoro-hCPTs derivative is one of the most promising new members of the 'tecan' family. This review summarizes the recent discoveries in the topoisomerase I field and presents the different camptothecin (CPT) analogues currently evaluated as anticancer agents. The specific properties of hCPTs are highlighted.

The mechanism of topoisomerase I poisoning by a camptothecin analog

We report the x-ray crystal structure of human topoisomerase I covalently joined to double-stranded DNA and bound to the clinically approved anticancer agent Topotecan. Topotecan mimics a DNA base pair and binds at the site of DNA cleavage by intercalating between the upstream (-1) and downstream (+1) base pairs. Intercalation displaces the downstream DNA, thus preventing religation of the cleaved strand. By specifically binding to the enzyme-substrate complex, Topotecan acts as an uncompetitive inhibitor. The structure can explain several of the known structure-activity relationships of the camptothecin family of anticancer drugs and suggests that there are at least two classes of mutations that can produce a drug-resistant enzyme. The first class includes changes to residues that contribute to direct interactions with the drug, whereas a second class would alter interactions with the DNA and thereby destabilize the drug-binding site.

Clinical pharmacokinetics of irinotecan and its metabolites: a population analysis

PURPOSE

To build population pharmacokinetic (PK) models for irinotecan (CPT-11) and its currently identified metabolites.

PATIENTS AND METHODS

Seventy cancer patients (24 women and 46 men) received 90-minute intravenous infusions of CPT-11 in the dose range of 175 to 300 mg/m(2). The PK models were developed to describe plasma concentration profiles of the lactone and carboxylate forms of CPT-11 and 7-ethyl-10-hydroxycamptothecin (SN-38) and the total forms of SN-38 glucuronide (SN-38G), 7-ethyl-10-[4-N-(5-aminopentanoic acid)-1-piperidino]-carbonyloxycamptothecin (APC), and 7-ethyl-10-[4-amino-1-piperidino]-carbonyloxycamptothecin (NPC) by using NONMEM.

RESULTS

The interconversion between the lactone and carboxylate forms of CPT-11 was relatively rapid, with an equilibration half-life of 14 minutes in the central compartment and hydrolysis occurring at a rate five times faster than lactonization. The same interconversion also occurred in peripheral compartments. CPT-11 lactone had extensive tissue distribution (steady-state volume of distribution [Vss], 445 L) compared with the carboxylate form (Vss, 78 L, excluding peripherally formed CPT-11 carboxylate). Clearance (CL) was higher for the lactone form (74.3 L/h) compared with the carboxylate form (12.3 L/h). During metabolite data modeling, goodness of fit indicated a preference of SN-38 and NPC to be formed out of the lactone form of CPT-11, whereas APC could be modeled best by presuming formation from CPT-11 carboxylate. The interconversion between SN-38 lactone and carboxylate was slower than that of CPT-11, with the lactone form dominating at equilibrium. The CLs for SN-38 lactone and carboxylate were similar, but the lactone form had more extensive tissue distribution.

CONCLUSION

Plasma data of CPT-11 and metabolites could be adequately described by this compartmental model, which may be useful in predicting the time courses, including interindividual variability, of all characterized substances after intravenous administrations of CPT-11.

Simple and efficient liposomal encapsulation of topotecan by ammonium sulfate gradient: stability, pharmacokinetic and therapeutic evaluation

Topotecan (TPT), a topoisomerase I inhibitor, is presently undergoing clinical evaluation worldwide. Previous studies have shown that entrapping TPT within multi-lamellar vesicle liposome can stabilize the lactone moiety, which is structurally important for biological activity. However, low drug:lipid ratios due to the amphipathic character and small entrapment volume in the unilamellar vesicle limits the development of pharmaceutically acceptable liposomal formulation. With an aim to improve on this drawback, we herein describe a method that utilizes the ammonium sulfate gradient to entrap TPT into liposomes. By this method, the encapsulation efficiency was over 90% and a drug:lipid molar ratio as high as 1:5.4 was reached. In comparison with free drug, liposome-encapsulated TPT is more stable in physiological conditions and shows higher in vitro cytotoxicity. Because of increased blood circulation time, the initial plasma concentration and area under the plasma concentration of liposomal drugs were 14 and 40 times, respectively, of those of free drug. Furthermore, liposome encapsulation enhanced the antitumor activity of TPT in syngeneic murine C-26 and human HTB-9 xenograft models in vivo. At a dose of 5 mg/kg, the tumor growth delay of liposomal formulation was significantly than that of free TPT. Based on these results, we believe that this liposomal TPT formulation is worthy of further clinical study.

Irinotecan pharmacokinetics-pharmacodynamics: the clinical relevance of prolonged exposure to SN-38

We have shown previously that the terminal disposition half-life of SN-38, the active metabolite of irinotecan, is much longer than earlier thought. Currently, it is not known whether this prolonged exposure has any relevance toward SN-38-induced toxicity. Here, we found that SN-38 concentrations present in human plasma for up to 3 weeks after a single irinotecan infusion induce significant cytotoxicity in vitro. Using pharmacokinetic data from 26 patients, with sampling up to 500 h, relationships were evaluated between systemic exposure (AUC) to SN-38 and the per cent decrease in absolute neutrophil count (ANC) at nadir, or by taking the entire time course of ANC into account (AOC). The time course of SN-38 concentrations (AUC(500 h)) was significantly related to this AOC (P<0.001). Based on these findings, a new limited-sampling model was developed for SN-38 AUC(500 h) using only two timed samples: AUC(500 h)=(6.588 x C(2.5 h))+(146.4 x C(49.5 h))+15.53, where C(2.5 h) and C(49.5 h) are plasma concentrations at 2.5 and 49.5 h after start of infusion, respectively. The use of this limited-sampling model may open up historic databases to retrospectively obtain information about SN-38-induced toxicity in patients treated with irinotecan.

Intestinal transport of irinotecan in Caco-2 cells and MDCK II cells overexpressing efflux transporters Pgp, cMOAT, and MRP1

Irinotecan (CPT-11) is a water-soluble camptothecin (CPT) derivative that has been recently approved in the United States for patients as a first-line therapy in advanced colorectal cancer. Phase I clinical trials using oral CPT-11 have shown poor and variable oral bioavailability. The present study was designed to investigate the intestinal absorption and efflux mechanisms of CPT-11 using in vitro cell culture models, Caco-2 cells, and engineered Madine-Darby canine kidney (MDCK) II cells overexpressing P-glycoprotein (Pgp), canalicular multispecific organic anion transporter (cMOAT), and multidrug resistance-associated protein (MRP1). The intestinal absorptive and secretory transport of CPT-11 was investigated using Caco-2 cell monolayers. Secretory transport was concentration-dependent and saturable. The secretory efflux permeability (P(eff)) of CPT-11 decreased with decreasing temperature, with an estimated activation energy of 19.6 +/- 2.9 kcal/mol suggesting the involvement of active transporters. The involvement of potential secretory transporters was further characterized in MDCK II cells. The secretory efflux carrier permeability (P(c)) was approximately 4- and approximately 2-fold greater in MDCK II/Pgp and MDCK II/cMOAT cells than that in MDCK II/wild-type cells. Furthermore, the secretory efflux P(eff) of CPT-11 was significantly decreased by Pgp inhibitors, elacridar (GF120918) (IC50 = 0.38 +/- 0.06 microM) and verapamil (IC(50) = 234 +/- 48 microM) in MDCK II/Pgp cells and by cMOAT inhibitor 3-([(3-(2-[7-chloro-2-quinolinyl]ethyl)phenyl]-[(3-dimethylamino-3-oxoprphyl)-thio)-methyl]-thio) propanoic acid (MK571) (IC50) = 469 +/- 60 micro;M) in MDCK II/cMOAT cells. Overall, the current study suggests that Pgp and cMOAT are capable of mediating the efflux of CPT-11 in vitro. Since both Pgp and cMOAT are expressed in the intestine, liver, and kidney, it is likely that these efflux transporters play a significant role limiting the oral absorption and disposition of this important anticancer drug.

An analytical method for irinotecan (CPT-11) and its metabolites using a high-performance liquid chromatography: parallel detection with fluorescence and mass spectrometry

Irinotecan or 7-ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin (CPT-11) is an anticancer pro-drug used in the treatment of many types of cancer. We describe here the validation of an analytical method for CPT-11 and its metabolites, including an active metabolite, 7-ethyl-10-hydroxycamptothecin (SN-38), its glucuronidated form, SN-38G, and several cytochrome P450 3A-mediated products such as 7-ethyl-10-[4-N-(5-aminopentanoic acid)-1-piperidino]carbonyloxycamptothecin (APC) using a high-performance liquid chromatography connected to parallel fluorescence and mass spectrometry detection systems. This method is characterized as follows: (1) simple extraction of the analytes from biomaterials with perchloric acid/methanol; (2) sensitive quantitation of major metabolites (SN-38G, SN-38 and APC) with a fluorescence detector (FLD), where the limits of quantitation by FLD were 2.5 ng/mL for SN-38G and APC, 5 ng/mL for CPT-11 and 1 ng/mL for SN-38, respectively; (3) parallel selective monitoring of the metabolites including minor metabolites with a mass selected detector (MSD). There was no observed interference by other drugs expected to be co-administered. This method showed its usefulness by identifying a novel metabolite produced in human hepatic microsomes. The results indicate that this combination of FLD and MSD enables a highly selective analysis of CPT-11 and its metabolites, and is useful for studies both in vivo and in vitro.

Phase I and pharmacologic study of liposomal lurtotecan, NX 211: urinary excretion predicts hematologic toxicity

PURPOSE

To determine the maximum-tolerated and recommended dose, toxicity profile, and pharmacokinetics of the liposomal topoisomerase I inhibitor lurtotecan (NX 211) administered as a 30-minute intravenous infusion once every 3 weeks in cancer patients.

PATIENTS AND METHODS

NX 211 was administered by peripheral infusion. Dose escalation decisions were based on all toxicities during the first cycle as well as pharmacokinetic parameters. Serial plasma, whole blood, and urine samples were collected for up to 96 hours after the end of infusion, and drug levels were determined by high-performance liquid chromatography.

RESULTS

Twenty-nine patients (16 women; median age, 56 years; range, 39 to 74 years) received 77 courses of NX 211 at dose levels of 0.4 (n = 3), 0.8 (n = 6), 1.6 (n = 3), 3.2 (n = 6), 3.8 (n = 6), and 4.3 mg/m(2) (n = 5). Neutropenia and thrombocytopenia were the dose-limiting toxicities and were not cumulative. Other toxicities were mild to moderate. Nine patients had stable disease while undergoing treatment. The systemic clearance of lurtotecan in plasma and whole blood was 0.82 +/- 0.78 L/h/m(2) and 1.15 +/- 0.96 L/h/m(2), respectively. Urinary recovery (Fu) of lurtotecan was 10.1% +/- 4.05% (range, 4.9% to 18.9%). In contrast to systemic exposure measures, the dose excreted in urine (ie, dose x Fu) was significantly related to the percent decrease in neutrophil and platelet counts at nadir (P <.00001).

CONCLUSION

The dose-limiting toxicities of NX 211 are neutropenia and thrombocytopenia. The recommended dose for phase II studies is 3.8 mg/m(2) once every 3 weeks. Pharmacologic data suggest a relationship between exposure to lurtotecan and NX 211-induced clinical effects.

The development of camptothecin analogs in childhood cancers

Camptothecin analogs, agents that target the intranuclear enzyme topoisomerase I, represent a promising new class of anticancer drugs for the treatment of childhood cancer. In preclinical studies, camptothecins, such as topotecan and irinotecan, are highly active against a variety of pediatric malignancies including neuroblastomas, rhabdomyosarcomas, gliomas, and medulloblastomas. In this paper, we review the status of completed and ongoing clinical trials and pharmacokinetic studies of camptothecin analogs in children. These and future planned studies of this novel class of cytotoxic agents are critical to defining the ultimate role of topoisomerase I poisons in the treatment of childhood cancer.

Camptothecins

Camptothecin analogues and derivatives appear to exert their antitumour activity by binding to topoisomerase I and have shown significant activity against a broad range of tumours. In general, camptothecins are not substrates for either the multidrug-resistance P-glycoprotein or the multidrug-resistance-associated protein (MRP). Because of manageable toxicity and encouraging activity against solid tumours, camptothecins offer promise in the clinical management of human tumours. This review illustrates the proposed mechanism(s) of action of camptothecins and presents a concise overview of current camptothecin therapy, including irinotecan and topotecan, and novel analogues undergoing clinical trails, such as exatecan (DX-8951f), IDEC-132 (9-aminocamptothecin), rubitecan (9-nitrocamptothecin), lurtotecan (GI-147211C), and the recently developed homocamptothecins diflomotecan (BN-80915) and BN-80927.