9-Aminocamptothecin by 72-hour continuous intravenous infusion is inactive in the treatment of patients with 5-fluorouracil-refractory colorectal carcinoma

Natural product inspired leads in the discovery of anticancer agents: an update

Natural products have emerged as major leads for the discovery and development of new anti-cancer drugs. The plant-derived anti-cancer drugs account for approximately 60% and the quest for new anti-cancer agents is in progress. Anti-cancer leads have been isolated from plants, animals, marine organisms, and microorganisms from time immemorial. The process of semisynthetic modifications of the parent lead has led to the generation of new anti-cancer agents with improved therapeutic efficacy and minimal side effects. The various chemo-informatics tools, bioinformatics, high-throughput screening, and combinatorial synthesis are able to deliver the new natural product lead molecules. Plant-derived anticancer agents in either late preclinical development or early clinical trials include taxol, vincristine, vinblastine, topotecan, irinotecan, etoposide, paclitaxel, and docetaxel. Similarly, anti-cancer agents from microbial sources include dactinomycin, bleomycin, mitomycin C, and doxorubicin. In this review, we highlighted the importance of natural products leads in the discovery and development of novel anti-cancer agents. The semisynthetic modifications of the parent lead to the new anti-cancer agent are also presented. Further, the leads in the preclinical settings with the potential to become effective anticancer agents are also reviewed.Communicated by Ramaswamy H. Sarma.

Topoisomerase 1B poisons: Over a half-century of drug leads, clinical candidates, and serendipitous discoveries

Topoisomerases are DNA processing enzymes that relieve supercoiling (torsional strain) in DNA, are necessary for normal cellular division, and act by nicking (and then religating) DNA strands. Type 1B topoisomerase (Top1) is overexpressed in certain tumors, and the enzyme has been extensively investigated as a target for cancer chemotherapy. Various chemical agents can act as "poisons" of the enzyme's religation step, leading to Top1-DNA lesions, DNA breakage, and eventual cellular death. In this review, agents that poison Top1 (and have thus been investigated for their anticancer properties) are surveyed, including natural products (such as camptothecins and indolocarbazoles), semisynthetic camptothecin and luotonin derivatives, and synthetic compounds (such as benzonaphthyridines, aromathecins, and indenoisoquinolines), as well as targeted therapies and conjugates. Top1 has also been investigated as a therapeutic target in certain viral and parasitic infections, as well as autoimmune, inflammatory, and neurological disorders, and a summary of literature describing alternative indications is also provided. This review should provide both a reference for the medicinal chemist and potentially offer clues to aid in the development of new Top1 poisons.

N-(2-Hydroxypropyl)methacrylamide Copolymer-9-Aminocamptothecin Conjugate: Colon-Specific Drug Delivery in Rats

An N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer conjugate containing 9-aminocamptothecin (9-AC) boundvia a spacer containing an aromatic azo bond and leucylalanine (P-Azo-Leu-Ala-9-AC) was synthesized. The in vivo pharmacokinetic profile after oral administration was examined in rats and comparedto free 9-AC. The aromatic azo bond of P-Azo-Leu-Ala-9-AC was stable in stomach and small intestine; the delivery of a large amount of intact conjugate to the colon was achieved. In the colon, the azoreductase activity first cleaved the azo bond followed by peptidase catalyzed cleavage of the leucylalanyl drug derivative resulting in the release of free 9-AC. However, the release rate from the conjugate was not fast enough to achieve high colon concentrations of free 9-AC. The results of the study suggest design features for the second generation of conjugates, including the use of a side-chain with a higher cleavage rate in the colon, combined with the incorporation of bioadhesion technology, to increase colon transit time.

Phase I trial of continuous infusion 9-aminocamptothecin in patients with advanced solid tumors: 21-day infusion is an active well-tolerated regimen

This study's objectives were to determine the maximum tolerated dose (MTD) of 9-aminocamptothecin (9-AC), given as a prolonged continuous infusion (CI) for 7-21 days, when formulated in dimethylacetamide/polyethylene glycol 400 (DMA) and then later as a colloidal dispersion (CD), and to determine the steady-state pharmacokinetics of 9-AC. Patients with solid tumors refractory to standard therapy were enrolled on this study. Total dose/cycle of 9-AC/DMA was initially escalated by duration (7-21 days), while keeping the dose rate constant at 6.2 microg/m/h (1.04-3.12 mg/m/4-week cycle). Then, the dose rate was escalated from 6.2 to 21.1 microg/m/h (3.12-10.6 mg/m/4-week cycle) while keeping the infusion duration constant at 21 days. CD formulation was escalated from 14.1 to 25 microg/m/h (7.11-12.60 mg/m/4-week cycle) while keeping the infusion duration constant at 21 days and then escalated from 28.1 to 37.5 microg/m/h (9.44-12.60 mg/m/3-week cycle) while keeping the infusion duration constant at 14 days. Sixty-two patients were evaluable for toxicity; 61 received prior chemotherapy (median 3 regimens/patient). No consistent dose-limiting toxicity (DLT) was encountered with the DMA formulation until dose level 10.60 mg/m/cycle, when two patients experienced DLTs. With the 21-day CD formulation, the MTD was 12.60 mg/m/cycle with three DLTs out of five patients. When 9-AC was given on the 14-day schedule, DLT was seen at 9.44, 11.20 and 12.60 mg/m/cycle, with consistent DLT at the two highest dose levels. All DLTs for both formulations were grade 4 hematologic toxicities (neutropenia and/or thrombocytopenia), while non-hematologic toxicities were relatively mild (including gastrointestinal toxicities and fatigue). One patient with ovarian cancer had a complete response and three had partial responses (PRs). One patient each with non-Hodgkin's lymphoma and cancer of unknown primary had a PR. Pharmacokinetic studies of both formulations of 9-AC revealed a linear relationship between increasing plasma 9-AC lactone concentration and dose. The median plasma 9-AC lactone concentration for 9-AC/CD was approximately twice that achieved by 9-AC/DMA for the same dose level. Both 9-AC formulations, given as a 21-day CI, were well tolerated with dose-limiting myelosupression at the MTD. This dose intensity exceeds that of other 9-AC phase I/II schedules. The recommended phase II dose (RPTD) is 9.42 mg/m/4-week cycle, given as a 21-day infusion. The 14-day schedule of 9-AC/CD was equally myelosuppressive with the RPTD of 9.44 mg/m/3-week cycle, although two heavily pre-treated patients (one with pelvic radiotherapy) could not tolerate this dose. Objective responses were observed in six out of 57 heavily pre-treated patients, most of which had ovarian cancer.

Phase II study of rubitecan, an oral camptothecin in patients with advanced colorectal cancer who have failed previous 5-fluorouracil based chemotherapy

BACKGROUND

Rubitecan (RFS-2000, 9NC, Orathecin) is an orally bioavailable camptothecin analogue, with evidence of preclinical activity in colon cancer cell lines. We evaluated oral rubitecan (5 days on, 2 days rest per week) on a continuous schedule, in patients with advanced colorectal cancer (CRC), who progressed after 5-fluorouracil based chemotherapy.

PATIENTS AND RESULTS

Fourteen eligible patients were treated with rubitecan at 1.5 mg/m2/day on a 5 day/week continuous schedule. Therapy was well tolerated with most adverse events in the mild to moderate category. Grade 3/4 toxicity consisting of anemia, diarrhea and elevated bilirubin was seen in 4 patients. No responses were seen in 13 evaluable patients. Overall median survival (95% confidence interval) was 10.1 (range 3.1-12.6) months, and median time to progression was 2.1 months.

CONCLUSIONS

Administration of rubitecan was well tolerated, but this schedule does not appear to have clinical activity in patients with advanced previously treated CRC.

Topoisomerase I Inhibitors in the Treatment of Gastrointestinal Cancer: From Intravenous to Oral Administration

This article reviews the current status of the topoisomerase I (top I) inhibitors in the treatment of gastrointestinal (GI) malignancies. We focus on oral drug administration, the mode of administration that is generally preferred by patients with cancer. However, the great majority of the studies have been performed with intravenous (I.V.) administration. The most extensively investigated GI malignancy in phase I/II studies is colorectal cancer (CRC), for which I.V. irinotecan is currently approved in the United States and Europe. We discuss the activity and efficacy of irinotecan as a single agent in CRC and in combination regimens. Also, results obtained with monotherapy and in combination treatment in other GI malignancies such as esophageal, gastric, and pancreatic cancer are discussed. Few phase I studies have been performed with oral irinotecan and its clinical activity has not yet been fully determined. Several top I inhibitors are discussed, including topotecan, 9-aminocamptothecin, rubitecan, exatecan, and lurtotecan. None of these agents, given orally or intravenously, have shown activity in CRC similar to that of I.V. irinotecan. However, several agents show promising results in other GI malignancies, eg, rubitecan and exatecan in pancreatic cancer. A complicating factor in the oral administration of the top I inhibitors is the often encountered low and variable oral bioavailability. This can partly be explained by the high affinity for the drug efflux pumps BCRP (ABCG2) and P-glycoprotein, which are highly expressed in the epithelial apical membrane of the GI tract. A novel approach to improve the oral bioavailability of the top I inhibitors by temporary blockade of the drug transporter BCRP is described.

Biweekly 72-Hour 9-Aminocamptothecin Infusion As Second-Line Therapy for Ovarian Carcinoma: Phase II Study of the New York Gynecologic Oncology Group and the Eastern Cooperative Oncology Group

Purpose

To determine the antitumor activity of the novel topoisomerase I inhibitor 9-aminocamptothecin (9-AC) given over 72 hours every 2 weeks in patients with ovarian carcinoma previously treated with one platinum-containing regimen.

Patients and Methods

Patients with ovarian carcinoma who received one prior platinum-containing regimen were eligible. Patients were stratified based on whether their disease was measurable, or nonmeasurable but assessable. 9-AC 35 μg/m2/h was administered by continuous infusion for 72 hours every 2 weeks via ambulatory pump.

Results

Sixty patients were entered, 32 with measurable and 28 with nonmeasurable but assessable disease. Ten (16.7%) of 60 patients responded (95% CI, 7.2% to 26.1%), with four complete responses and six partial remissions. The response rate for patients with measurable and nonmeasurable but assessable disease was 22% (95% CI, 7.6% to 36.2%) and 10.7% (95% CI, 2.3% to 28.2%), respectively. None of the responders were platinum-resistant. Nineteen patients (32%) had stable disease. The major toxicities were hematologic, with 25% of patients having grade 3 and 35% having grade 4 neutropenia, including five episodes of febrile neutropenia, 17% having grade 3 to 4 thrombocytopenia, and 27% having grade 3 to 4 anemia. Nonhematologic toxicity included grade 3 to 4 nausea (27%) and grade 3 to 4 vomiting (12%).

Conclusion

This phase II multicenter trial of biweekly 72 hour 9-AC infusion as second-line therapy for ovarian cancer demonstrates comparable activity to standard approved agents in patients with both measurable and nonmeasurable but assessable disease. Toxicity consists mainly of moderate but controllable myelosuppression. Further studies combining 9-AC with other agents active in ovarian cancer for use as second-line therapy are warranted.

The camptothecins

Supported by detailed understanding of their mechanism of action, and facilitated by chemical manipulations that have amplified their solubility, the camptothecins have advanced to the forefront of several areas of therapeutic and developmental chemotherapy. Additive and synergistic laboratory interactions with other cytotoxic drugs have been exploited to allow development of camptothecin-based multidrug regimens, which are showing important activity in several malignancies. Topotecan and irinotecan are already in widespread use in clinical practice, and newer agents with promising preclinical activity are in various stages of clinical assessment. As knowledge of molecular and biochemical mechanisms of action and resistance continues to expand, newer and better camptothecin-based strategies for treatment of malignant disease are likely to evolve.

A phase II trial of 9-aminocaptothecin (9-AC) as a 120-h infusion in patients with non-small cell lung cancer

In a previous phase II trial of the synthetic topoisomerase I inhibitor, 9-aminocamptothecin (9-AC), given as a 72-h infusion, we identified modest single agent activity of 9% in patients with previously untreated advanced non-small cell lung cancer (NSCLC). Preclinical studies suggested that a more prolonged continuous infusion of the drug might lead to greater antitumor activity. A phase I study recommended a phase II dose of 25 microg/m2/hr for 120 h (3000 microg/m2 over 5 days), administered for 2 consecutive weeks of a 3-week cycle. We utilized this schedule and enrolled 13 chemotherapy-naïve patients with Stage IIIB and IV NSCLC in this trial: median age 67 (range 57-74); 46% male; 92% stage IV; and median performance status 1. Twelve patients are available for response and toxicity evaluation after 2 cycles of therapy. One patient achieved a partial response. Four patients had stable disease while seven patients had progressive disease. Patients with stable or progressive disease after two cycles received no additional 9-AC, and were offered conventional chemotherapy. The median survival time was 10.2 months and the one-year survival rate 28% (95% confidence interval, 5-58%). Significant toxicities included myelosuppression, fatigue, and anorexia. One patient had grade 4 neutropenia following the first week of cycle 2, and did not receive additional therapy. There were no neutropenia-related infections. These data suggest that this prolonged schedule is unlikely to increase 9-AC's very modest activity in NSCLC above that seen with the simpler 72-h administration schedule. Further evaluation of 9-AC in NSCLC is not recommended.

Biorecognizable HPMA copolymer-drug conjugates for colon-specific delivery of 9-aminocamptothecin

N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer conjugates for colon-specific delivery of 9-aminocamptothecin (9-AC) were designed. They hold 9-AC bound via spacers containing amino acid residues and aromatic azo bonds. In vitro release profiles of 9-AC from HPMA copolymer conjugates were evaluated under artificial conditions that simulated large intestinal azoreductase and peptidase activities. The studies indicated that the azo bond was reduced first, followed by the release of unmodified 9-AC from the 9-AC containing fragment by peptidases. Release profiles depended on the chemical structure of the peptide part of the spacer. Conjugates containing leucylalanine showed high colon-specific release of 9-AC when compared to alanine containing conjugates. It appears that the studied conjugates are suitable as colon-specific drug delivery systems.

The clinical development of 9-aminocamptothecin

9-Aminocamptothecin (9-AC) is a topoisomerase I-targeting agent first synthesized by Wani and Wall in 1986. Because of its potent in vitro effects and promising preclinical activity in colorectal cancer animal models, it was designated a high-priority compound for further drug development by the NCI. In 1993, 9-AC first entered clinical trials as a 72-hour intravenous (i.v.) infusion. Predictable myelosuppression was the major dose-limiting toxicity, and pharmacokinetic studies showed a relatively short plasma half-life and unstable lactone ring. Unfortunately, phase II studies using this schedule showed minimal or no activity in tumors such as colorectal and lung cancer. Modest activity was observed in ovarian cancer and in refractory lymphomas. Efforts to improve systemic drug exposure by utilizing alternative schedules of administration of 9-AC such as prolonged, continuous intravenous infusions have also been tested. However, phase II studies of 120-hour weekly infusions of 9-AC have not shown improved activity against solid tumors such as colorectal cancer. More recently, a daily times 5 days i.v. administration schedule has been tested. Currently, further development of intravenously administered 9-AC for the treatment of colorectal cancer is not promising. Thus, topotecan and irinotecan remain the only two successfully developed topoisomerase I-targeting drugs in the United States. This experience with 9-AC raises important questions regarding how to best select new topoisomerase I-targeting drugs for future clinical development.

Modulation of camptothecin analogs in the treatment of cancer: a review

The topoisomerase I inhibitors reviewed in this paper are all semisynthetic analogs of camptothecin (CPT). Modulation of this intranuclear enzyme translates clinically in to antitumor activity against a broad spectrum of tumors and is therefore the subject of numerous investigations. We present preclinical and clinical data on CPT analogs that are already being used in clinical practice [i.e. topotecan and irinotecan (CPT-11)] or are currently in clinical development (e.g. 9-aminocamptothecin, 9-nitrocamptotecin, lurtotecan, DX 8951f and BN 80915), as well as drugs that are still only developed in a preclinical setting (silatecans, polymer-bound derivates). A variety of different strategies is being used to modulate the systemic delivery of this class of agents, frequently in order to increase antitumor activity and/or reduce experienced side effects. Three principal approaches are discussed, including: (i) pharmaceutical modulation of formulation vehicles, structural alterations and the search for more water-soluble prodrugs, (ii) modulation of routes of administration and considerations on infusion duration, and (iii) both pharmacodynamic and pharmacokinetic biomodulation.

Oral chemotherapeutic agents for colorectal cancer

A number of novel oral chemotherapeutic agents are entering practice or are under development in the United States. Many of these agents display significant clinical activity against colorectal cancer. Many classes of compounds, including fluoropyrimidine analogs, dihydropyrimidine dehydrogenase (DPD) inhibitors, topoisomerase inhibitors, farnesyl transferase inhibitors, and others, are being developed for oral administration. This manuscript describes the progress of clinical development of these agents and also explores the relative merits and challenges of these approaches. Economic issues, patient preference, and patient selection issues surrounding oral chemotherapy for colorectal cancer will also be discussed.

Topoisomerase enzymes as drug targets

DNA topoisomerases catalyze changes in the topology of DNA. Recently, other functions have also been reported for these enzymes. For example, topoisomerase I participates in transcription by RNA polymerases I, II, and III, and also has a kinase activity. Topoisomerase I binds directly to at least two helicases, nucleolin and SV40 T antigen, and mechanistic studies show that T antigen alters the function of topoisomerase I. Additional protein and nucleotide interactions for both topoisomerases I and II suggest that each protein is multifunctional. It may be that the multifunctional nature of these enzymes is the basis for the antitumor activity seen with inhibitors of these enzymes. Clinical trials with combinations of CPT-11 and 5-fluorouracil for the treatment of colon cancer, and preclinical studies with CPT-11 and vincristine are particularly encouraging. Protracted schedules of administration of topoisomerase inhibitors will likely have greater antitumor effect than more concentrated, higher dose exposures, but a systematic determination of optimal schedules of administration is needed.

Human DNA topoisomerase I: quantitative analysis of the effects of camptothecin analogs and the benzophenanthridine alkaloids nitidine and 6-ethoxydihydronitidine on DNA topoisomerase I-induced DNA strand breakage

Human DNA topoisomerase I (topo I) has been purified from normal placenta and from a recombinant baculovirus expression system. A new radiolabeled plasmid DNA assay has been used to quantitate the activity of the purified enzymes and to compare the ability of several types of topo I-targeted drugs to induce topo I-mediated DNA strand breaks. The 100-kDa recombinant enzyme form isolated from the baculovirus expression system is able to relax 2564 ng of supercoiled M-13 mp19 plasmid per minute per nanogram of enzyme. The addition of camptothecin (1 microM) to the reaction lowers the rate to 1282 ng per minute per nanogram of enzyme. The 100-kDa topo I from human placenta is able to relax 1092 ng of supercoiled plasmid per minute per nanogram of enzyme and the 68-kDa topo I form from placenta is able to relax 2069 ng of supercoiled plasmid per minute per nanogram of enzyme. Camptothecin (1 microM) decreases the relaxation rate of the placental enzymes about 50%. In the presence of several different types of topo I-targeted drugs, both the recombinant and placental enzymes are induced to cleave plasmid DNA. Quantitative DNA cleavage assays with radioactive plasmid DNA and 9-aminocamptothecin, topotecan, SN-38, 10, 11-methylenedioxycamptothecin, 7-ethyl-10, 11-methylenedioxycamptothecin, 7-chloromethyl-10, 11-methylenedioxycamptothecin, nitidine, and 6-ethoxy-5, 6-dihydronitidine indicate that the order of potency in inducing topo I-mediated DNA breakage is methylenedioxycamptothecin analogs > SN-38 > 9-aminocamptothecin > topotecan and camptothecin > nitidine compounds. The order of potency correlates with the half-lives of the topo I-DNA drug complex determined with radiolabeled DNA in 0.45 M NaCl at 30 degrees C. The half-life of the complex formed with 7-chloromethyl-10,11-methylenedioxycamptothecin is greater than 90 min whereas the half-life of the topo I-DNA complex with 6-ethoxy-5, 6-dihydronitidine is less than 15 s. The other drugs tested were found to have drug complex half-lives which fall between these two extremes.

Phase I and pharmacokinetic study of a daily times 5 short intravenous infusion schedule of 9-aminocamptothecin in a colloidal dispersion formulation in patients with advanced solid tumors

PURPOSE

To determine the maximum-tolerated dose (MTD), dose-limiting toxicities (DLT), and pharmacokinetics of 9-aminocamptothecin (9-AC) in a colloidal dispersion (CD) formulation administered as a 30-minute intravenous (IV) infusion over 5 consecutive days every 3 weeks.

PATIENTS AND METHODS

Patients with solid tumors refractory to standard therapy were entered onto the study. The starting dose was 0.4 mg/m(2)/d. The MTD was assessed on the first cycle and was defined as the dose at which > or = two of three patients or > or = two of six patients experience DLT. Pharmacokinetic measurements were performed on days 1 and 5 of the first cycle and on day 4 of subsequent cycles using high-performance liquid chromatography.

RESULTS

Thirty-one patients received 104+ treatment courses at seven dose levels. The DLT was hematologic. At a dose of 1.3 mg/m(2)/d, three of six patients experienced grade 3 thrombocytopenia. Grade 4 neutropenia that lasted less than 7 days was observed in four patients. At a dose of 1.1 mg/m(2)/d, four of nine patients had grade 4 neutropenia of brief duration, which was not dose limiting. Nonhematologic toxicities were relatively mild and included nausea/vomiting, diarrhea, obstipation, mucositis, fatigue, and alopecia. Maximal plasma concentrations and area under the concentration-time curve (AUC) increased linearly with dose, but interpatient variation was wide. Lactone concentrations exceeded 10 nmol/L, the threshold for activity in preclinical tumor models, at all dose levels. Sigmoidal E(max) models could be fit to the relationship between AUC and the degree of hematologic toxicity. A partial response was observed in small-cell lung cancer.

CONCLUSION

9-AC CD administered as a 30-minute IV infusion daily times 5 every three weeks is safe and feasible. The recommended phase II dose is 1. 1 mg/m(2)/d.

A phase II study of 9-aminocamptothecin in advanced non-small-cell lung cancer

BACKGROUND

9-Aminocamptothecin (9-AC) is a synthetic analogue of camptothecin. Phase I studies, identified the maximum tolerated dose as 1416 micrograms/m2/day x 3 as continuous intravenous infusion (CVI) with dose-limiting neutropenia.

PATIENTS AND METHODS

Eligible patients had stage IIIB or IV non-small-cell lung cancer (NSCLC) with measurable disease. Patients were initially treated at 1416 micrograms/m2/d x 3 by CVI followed by granulocyte-colony stimulating factor (G-CSF) support. This dose was decreased to 1100 micrograms/m2/d after the first 13 patients. Cycles were repeated every 14 days until tumor progression.

RESULTS

Fifty-eight patients were treated, thirteen at 1416 micrograms/m2/d and 45 at 1100 micrograms/m2/d. Fifty percent had adenocarcinoma and 17% squamous cell carcinoma. Seventy-one percent had stage IV disease. Five patients had a partial response (response duration 9-28 weeks) for an overall response rate of 8.6%, (95% confidence intervals (CI): 2.9%-19%). Median time to progression was 2.3 months and the median survival for the entire study population 5.4 months with a one-year survival rate of 30%. The one-year survival rate for 27 patients who received second line chemotherapy was 56.7%. Toxicities at 1416 micrograms/m2/d included grade 4 neutropenia and thrombocytopenia in six and five of 13 patients, respectively; at 1100 micrograms/m2/d these toxicities were observed in 12 and three of 45 patients, respectively.

CONCLUSION

9-AC has modest single-agent activity in previously untreated NSCLC. Its further evaluation at the dose and schedule employed in this study does not seem indicated. Exploration of more prolonged administration schedules may be warranted.