Camptothecins

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.

A phase I clinical and pharmacokinetic study of the camptothecin glycoconjugate, BAY 38-3441, as a daily infusion in patients with advanced solid tumors

BACKGROUND

The aim of this study was to define the maximum tolerated dose (MTD), dose-limiting toxicity (DLT) and pharmacokinetics of the camptothecin glycoconjugate BAY 38-3441, administered as an infusion for 30 min on two separate schedules every 3 weeks.

PATIENTS AND METHODS

A total of 81 patients with advanced solid tumors were treated with BAY 38-3441 either at doses of 20, 40, 67, 100, 140, 210, 315, 470 and 600 mg/m2/day for 1 day every 3 weeks (single-dose schedule), or at doses of 126, 189, 246, 320 and 416 mg/m2/day once daily for three consecutive days every 3 weeks (3-day schedule). Plasma sampling was performed to characterize the pharmacokinetics of BAY 38-3441 and camptothecin with these schedules.

RESULTS

DLTs included renal toxicity, granulocytopenia and thrombocytopenia on the single-day schedule at doses > or = 470 mg/m2/day, and diarrhea and thrombocytopenia on the 3-day schedule at doses > or = 320 mg/m2/day. Other non-DLTs were gastrointestinal, dermatological and hematological. Pharmacokinetics of BAY 38-3441 and camptothecin appear to be dose-dependent, but not linear.

CONCLUSIONS

Renal toxicity was dose-limiting for BAY 38-3441 using 30-min infusions on the single-dose schedule. Dose escalation to 470 mg/m2/day is feasible using a 2-h infusion. However, because of the superior safety profile, we recommend the 3-day schedule for BAY 38-3441 at a dose of 320 mg/m2/day as 30-min infusions for further phase II studies.

Topoisomerases of kinetoplastid parasites as potential chemotherapeutic targets

The protozoan parasites Trypanosoma, Leishmania and Crithidia, which belong to the order kinetoplastidae, emerge from the most ancient eukaryotic lineages. The diversity found in the life cycle of these organisms must be directed by genetic events, wherein topoisomerases play an important role in cellular processes affecting the topology and organization of intracellular DNA. Topoisomerases are valuable as potential drug targets because they have indispensable function in cell biology. This review summarizes what is known about topoisomerase genes and proteins of kinetoplastid parasites and the roles of these enzymes as targets for therapeutic agents.

Novel vitamin E analogue and 9-nitro-camptothecin administered as liposome aerosols decrease syngeneic mouse mammary tumor burden and inhibit metastasis

PURPOSE

To test the anticancer properties of a nonhydrolyzable ether-linked acetic acid analogue of vitamin E, 2,5,7,8-tetramethyl-2R-(4R,8R, 12-trimethyltridecyl)chroman-6-yloxyacetic acid (alpha-TEA), and a derivative of camptothecin, 9-nitrocamptothecin (9-NC)singly and in combination against mouse mammary tumor cells (line 66 clone 4 stably transfected with green fluorescent protein; 66cl-4-GFP) cultured in vitro or transplanted subcutaneously into the inguinal region of female BALB/c mice to form established tumors.

METHODS

Following in vitro treatment of 66cl-4-GFP cells with alpha-TEA and suboptimal concentrations of 9-NC, singly or in combination, apoptosis was measured by morphological evaluation of nuclei stained with 4',6-diamidino-2-phenylindole (DAPI), and DNA synthesis arrest was measured by tritiated thymidine uptake. For in vivo analyses alpha-TEA and 9-NC, both water-insoluble compounds, were formulated into liposomes using dil-auroylphosphatidylcholine and administered by aerosol to deliver doses calculated to be 36 and 0.4 microg/mouse per day, respectively, (singly or each separately for combined treatments) 7 days per week.

RESULTS

Treatment of 66cl-4-GFP cells in culture for 3 days with a combination of alpha-TEA (10 microg/ml; singly produces 38% apoptosis), and suboptimal concentrations of 9-NC(15.6, 31.3, 62.5, or 125 ng/ml; singly produce 2-7% apoptosis), produced 47%, 58%, 64%, and 69% apoptosis. Likewise, combinations of alpha-TEA 9-NC inhibited DNA synthesis more than either agent administered singly. A significant reduction (P< 0.001)in growth of subcutaneous transplanted tumors was observed with liposome-formulated and aerosolized delivery of alpha-TEA + 9-NC to BALB/c mice. The incidence of macroscopic lung metastasis was 83% in control vs 8 % in alpha-TEA-, 9-NC-, or combination-treated mice. Fluorescence microscopic examination of lungs and axillary and brachial lymph nodes showed a statistically significant decrease in metastasis observed in alpha-TEA-,9-NC-, and combination- vs control-treated animals. Analyses of primary tumor tissue for proliferation and apoptosis showed treatment groups to have lower Ki-67 and higher terminal deoxynucleotidyl transferase-mediated nick end labeling, respectively. Treatments showed no measurable effects on two angiogenesis parameters,namely intratumoral blood volume as assessed by hemoglobin content and intratumoral blood vessel density as assessed with CD31 staining.

CONCLUSIONS

Combination treatments enhanced antiproliferative and proapoptotic activities in cell culture, and when formulated in liposomes and delivered via aerosolization to treat an aggressive and metastatic syngeneic murine mammary tumor, the combination treatment showed a significant reduction in tumor volume in comparison to either treatment alone. Mechanistically, it appears that neither enhanced apoptosis, reduced cell proliferation,nor reduced blood vessel density can fully account for the enhanced effects of the combination treatment.

An evaluation of transmembrane ion gradient-mediated encapsulation of topotecan within liposomes

Topotecan can be encapsulated in liposomes, however little is known about the role encapsulated counter ions play in drug loading efficiency and drug release. Using 1,2-distearoyl-sn-glycero-3 phosphatidylcholine and cholesterol liposomes (55:45 mole ratio), encapsulation was achieved using manganese ion gradients (MnSO(4) or MnCl(2)), with the addition of A23187, a divalent cation/proton exchanger, to maintain a pH gradient. This methodology was compared to procedures where the pH gradient was generated by use of encapsulated (NH(4))(2)SO(4) or citrate (300 mM, pH 3.5). All methods facilitated topotecan encapsulation. Liposomes prepared in the presence of the citrate and MnCl(2) (+A23187) exhibited reduced loading capacities. Liposomes prepared in the presence of (NH(4))(2)SO(4) and MnSO(4) (+A23187) could be used to generate liposomes exhibiting a drug-to-lipid ratio of 0.3 (wt/wt) with an encapsulation efficiency of >90%. In vitro drug release data suggested that the (NH(4))(2)SO(4) and MnSO(4) (+A23187) formulations released drug at a reduced rate. For these formulations, the drug release rates decreased as the drug-to-lipid ratio (wt/wt) increased from 0.1 to 0.2. Cryo-electron micrographs indicated that encapsulated topotecan precipitated as linear particles within liposomes. The stability of topotecan loaded liposomes appeared to be dependent on the presence of both a pH gradient and encapsulated sulfate.

Synthesis and biochemical properties of E-ring modified luotonin A derivatives

Luotonin A is a cytotoxic pyrroloquinazolinoquinoline alkaloid that has been shown to stabilize the human topoisomerase I-DNA covalent binary complex in the same fashion as the antitumor alkaloid camptothecin. A study of the structural elements in luotonin A required for binary complex stabilization has revealed key differences relative to those required for camptothecin.

Phase II study of topotecan and paclitaxel for patients with previously untreated extensive stage small-cell lung cancer

BACKGROUND

This phase II study was conducted to evaluate the safety and efficacy of the combination of paclitaxel and topotecan for patients with extensive stage small-cell lung cancer (ED-SCLC).

PATIENTS AND METHODS

Previously untreated ED-SCLC patients with Eastern Coperative Oncology Group performance status <2 were eligible. Treatment consisted of topotecan 1 mg/m2 (first three patients received 1.25 mg/m2), on days 1-5, and paclitaxel 135 mg/m2 over 24 h on day 5, every 4 weeks. Prophylactic granulocyte colony-stimulating factor was administered to all patients.

RESULTS

Thirty-two patients received a median of four cycles of chemotherapy. Grade 4 anemia, neutropenia and thrombocytopenia occurred in 13, 31 and 18 patients, respectively. Thirty episodes of febrile neutropenia occurred in 22 patients. Grade 3 fatigue, esophagitis, stomatitis and hypotension occurred in one patient each. Of 26 patients eligible for response evaluation, there were six complete and 12 partial responses (overall response rate 69%). The median survival was 54 weeks. The 1-, 2- and 3-year survival rates were 50%, 10% and 3%, respectively.

CONCLUSIONS

The combination of topotecan and paclitaxel is active as initial therapy in SCLC, but the efficacy is similar to 'standard therapy'. This combination was associated with a high incidence of myelosuppression and febrile neutropenia, at the doses evaluated.

5H-8,9-dimethoxy-5-(2-N,N-dimethylaminoethyl)dibenzo[c,h][1,6]naphthyridin-6-ones and related compounds as TOP1-targeting agents: influence of structure on the ternary cleavable complex formation

In this paper, we present our results from a docking study of the title compounds with the DNA/topoisomerase I complex based on the recently published X-ray crystal structure of the topotecan/DNA/topoisomerase I ternary cleavable complex (Staker, B.L., et al. PNAS 2002, 99, 15387) using the Autodock program. Simple intermolecular docking energies (E(dock)) correlate well with in vitro DNA cleavage data suggesting that the binding mode from the crystal structure is a reasonable binding mode for these compounds.

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.

Therapeutic approaches to bladder cancer: identifying targets and mechanisms

Transitional cell carcinoma is the second most common genitourinary malignancy in US and third most common cause of death among genitourinary tumors. Treatment options for bladder cancer include surgery, often combined with chemotherapy, radiation, and/or immunotherapy. The MVAC adjuvant chemotherapy regimen has been most widely used in locally invasive as well as metastatic disease. Only a proportion of patients at risk will respond to therapy. There is thus need to identify good responder patients for adjuvant therapy and to identify new targets to treat a greater range of patients. Based upon patient-specific aberrations in pathways or known markers, both existing and new therapies can be tailored to benefit patients based on the risk of progression and molecular alterations specific to a patient's tumor. Targeted therapy, therefore, is defined as therapy that targets mechanism and risk. Utilizing the available knowledge of the molecular biology of cell-cycle regulation, signal transduction, apoptosis, and angiogenesis in bladder cancer, we review the potential therapeutic targets for rational drug development. Finally, using bladder cancer as a model for translational research, requirements for a desired clinical trial are presented.