Review Camptothecin: Current Perspectives

An HPLC assay for the lipophilic camptothecin analog AR-67 carboxylate and lactone in human whole blood

AR-67 (7-t-butyldimethylsilyl-10-hydroxycamptothecin, DB-67) is a camptothecin analog currently in early stage clinical trials. The lactone moiety of camptothecins hydrolyzes readily in blood to yield the pharmacologically inactive carboxylate form. However the lactone form of third-generation lipophilic congeners, such as AR-67, is more stable, possibly due to partitioning into red cell membranes. This prompted us to develop a reverse-phase HPLC method with fluorescence detection (excitation 380 nm/emission 560 nm), which could quantitate the concentration of AR-67 lactone and carboxylate in whole blood. Samples were prepared by red cell lysis, protein precipitation with methanol and centrifugation to remove denatured materials. Recovery was estimated to be >85%. Analytes were eluted isocratically with 0.15 m ammonium acetate buffer containing 10 mm TBAP (pH 6.5) and acetonitrile (65:35, v/v) on a Nova-Pak C(18) column (4 µm; 3.9 × 150 mm). The assay was linear in the ranges 0.5-300 and 2.5-300 ng/mL for carboxylate and lactone, respectively. Accuracy and precision were acceptable. AR-67 forms were stable in whole blood and in methanolic supernatants. This assay has been successfully applied to measure AR-67 concentrations in whole blood of patients enrolled in a phase I study.

Selection of evodiamine as a novel topoisomerase I inhibitor by structure-based virtual screening and hit optimization of evodiamine derivatives as antitumor agents

Human topoisomerase I (TopoI) is recognized as a valuable target for the development of effective antitumor agents. Structure-based virtual screening was applied to the discovery of structurally diverse TopoI inhibitors. From 23 compounds selected by virtual screening, a total of 14 compounds were found to be TopoI inhibitors. Five hits (compounds 1, 14, 20, 21, and 23) also showed moderate to good in vitro antitumor activity. These novel structures can be considered as good starting points for the development of new antitumor lead compounds. Hit 20 (evodiamine) was chosen for preliminary structure-activity relationship studies. Various groups, including alkyl, benzoyl, benzyl and ester, were introduced to the indole nitrogen atom of evodiamine. The substituted benzoyl groups were found to be favorable for the antitumor activity and spectrum. The 4-Cl benzoyl derivative, compound 29u, was the most active one with IC(50) values in the range 0.049-2.6 μM.

PAMAM-camptothecin conjugate inhibits proliferation and induces nuclear fragmentation in colorectal carcinoma cells

PURPOSE

To synthesize and characterize a poly (amido amine) dendrimer-camptothecin (PAMAM-CPT) conjugate and evaluate its activity on human colorectal carcinoma cells (HCT-116).

METHODS

The attachment of CPT to amine-terminated PAMAM was facilitated through a succinic acid-glycine linker. The conjugate was characterized for absence of small molecular weight impurities, size and drug content. Stability of the conjugate in PBS and growth media and its in vitro activity on HCT-116 were studied. Cell cycle arrest and nuclear fragmentation upon PAMAM-CPT treatment were investigated.

RESULTS

The conjugate was stable under physiological pH (7.4) in PBS and in growth media (with 10% FBS) with minimal release of 4% and 6% drug, respectively, at 48 h. PAMAM-CPT inhibited proliferation of HCT-116 cells with an IC50 value of 1.6  ±  0.3 µM. The conjugate induced signs of cell cycle arrest with up to 68% of cells blocked in the G(2) phase. Confocal images of cells treated with PAMAM-CPT suggest nuclear fragmentation and formation of apoptotic bodies.

CONCLUSIONS

Results show that the PAMAM-CPT conjugate was active against colorectal cancer cells in vitro, inhibiting their growth and inducing nuclear fragmentation. Coupled with the ability to target macromolecular therapeutics to tumors, this conjugate shows promise for cancer chemotherapy.

The use of endophytes to obtain bioactive compounds and their application in biotransformation process

Endophytes are microorganisms that reside asymptomatically in the tissues of higher plants and are a promising source of novel organic natural metabolites exhibiting a variety of biological activities. The laboratory of Bioaromas (Unicamp, Brazil) develops research in biotransformation processes and functional evaluation of natural products. With the intent to provide subsidies for studies on endophytic microbes related to areas cited before, this paper focuses particularly on the role of endophytes on the production of anticancer, antimicrobial, and antioxidant compounds and includes examples that illustrate their potential for human use. It also describes biotransformation as an auspicious method to obtain novel bioactive compounds from microbes. Biotransformation allows the production of regio- and stereoselective compounds under mild conditions that can be labeled as "natural," as discussed in this paper.

Structure-based design, synthesis, and biological studies of new anticancer norindenoisoquinoline topoisomerase I inhibitors

On the basis of the superimposition of the crystal structures of norindenoisoquinoline 5 and topotecan (2) bound in the topoisomerase I-DNA covalent complex, as well as molecular docking and quantum chemical calculations, the substituted norindenoisoquinoline 14a was designed by transporting the 9-dimethylaminomethyl group of topotecan to the 10-position of the norindenoisoquinoline 5. The desired compound 14a was synthesized and found to possess topoisomerase I inhibitory activity that was slightly better than that of the starting compound 5. A focused set of 10-substitued norindenoisoquinoline analogues were then synthesized. The imidazole-substituted compound 14c was highly cytotoxic when evaluated in a series of human leukemia, ovarian, and breast cancer cells.

2-Ethyl-6-(2-pyrid-yl)-5,6,6a,11b-tetra-hydro-7H-indeno[2,1-c]quinoline

The title compound, C(23)H(22)N(2), was obtained using the three-component imino Diels-Alder reaction via a one-pot condensation between anilines, α-pyridine-carboxy-aldehyde and indene using BF(3)·OEt(2) as the catalyst. The mol-ecular structure reveals the cis-form as the unique diastereoisomer. The crystal structure comprises one-dimensional zigzag ribbons connected via N-H⋯N hydrogen bonds. C-H⋯π inter-actions also occur.

Organocatalytic cascade reactions as a new tool in total synthesis

The total synthesis of natural products and biologically active compounds, such as pharmaceuticals and agrochemicals, has reached an extraordinary level of sophistication. We are, however, still far away from the 'ideal synthesis' and the state of the art is still frequently hampered by lengthy protecting-group strategies and costly purification procedures derived from the step-by-step protocols. In recent years several new criteria have been brought forward to solve these problems and to improve total synthesis: atom, step and redox economy or protecting-group-free synthesis. Over the past decade the research area of organocatalysis has rapidly grown to become a third pillar of asymmetric catalysis standing next to metal and biocatalysis, thus paving the way for a new and powerful strategy that can help to address these issues - organocatalytic cascade reactions. In this Review we present the first applications of such asymmetric organocascade reactions to the total synthesis of natural products.

Quinoline alkaloids as intercalative topoisomerase inhibitors

Quinoline alkaloids are abundant in the Rutaceae, and many have exhibited cytotoxic activity. Because structurally related antitumor alkaloids such as camptothecin and fagaronine are known to function as intercalative topoisomerase poisons, it is hypothesized that cytotoxic Stauranthus alkaloids may also serve as intercalative topoisomerase inhibitors. To test this hypothesis theoretically, ten Stauranthus quinoline alkaloids were examined for potential intercalation into DNA using a molecular docking approach. Four of the alkaloids (stauranthine, skimmianine, 3',6'-dihydroxy-3',6'-dihydrostauranthine, and trans-3',4'-dihydroxy-3',4'-dihydrostauranthine) were able to intercalatively dock consistently into DNA. In order to probe the intermolecular interactions that may be responsible for intercalation of these quinoline alkaloids, density functional calculations have been carried out using both the B3LYP and M06 functionals. M06 calculations indicated favorable pi-pi interactions between either skimmianine or stauranthine and the guanine-cytosine base pair. Furthermore, the lowest-energy face-to-face orientation of stauranthine with guanine is consistent with favorable dipole-dipole orientations, favorable electrostatic interactions, and favorable frontier molecular orbital interactions. Likewise, the lowest-energy face-to-face orientation of stauranthine with the guanine-cytosine base pair reveals favorable electrostatic interactions as well as frontier molecular orbital interactions. Thus, not only can quinoline alkaloids dock intercalatively into DNA, but the docked orientations are also electronically favorable.

Molecular cytogenetic evaluation of the mechanism of micronuclei formation induced by camptothecin, topotecan, and irinotecan

We used the conventional bone marrow micronucleus test complemented with the fluorescent in situ hybridization with the minor satellite DNA probe to investigate the mechanisms of induction of micronuclei in mice treated with camptothecin and its clinical antineoplastic analogues topotecan and irinotecan. All experiments were performed with male Swiss albino mice. Single doses of 1 mg/kg camptothecin or 0.6 mg/kg topotecan were injected intraperitoneally and bone marrow was sampled at 30 hr (camptothecin) or 24 hr (topotecan) after treatment. A dose of 60 mg/kg irinotecan was injected intravenously, once every fourth day for 13 days and bone marrow was sampled 24 hr after the last treatment. In animals treated with camptothecin, a total of 1.07% micronuclei were found and 70% of them were centromere-negative, indicating their formation by DNA strand breaks and reflecting the predominant clastogenic activity of camptothecin. Exposure to topotecan and irinotecan yielded 1.71 and 0.83% micronuclei, respectively. About 52.7 and 48.8% of the induced micronuclei, respectively, were centromere-positive, indicating their formation by whole chromosomes and reflecting the aneugenic activity of both compounds. Correspondingly, about 47.3 and 51.2% of the induced micronuclei, respectively were centromere-negative, demonstrating that topotecan and irinotecan not only induce chromosome loss but also DNA strand breaks. Both the clastogenic and aneugenic potential of these drugs can lead to the development of secondary tumors and abnormal reproductive outcomes. Therefore, the clinical use of these agents must be weighed against the risks of secondary malignancies in cured patients and persistent genetic damage of their potential offspring.

Catalyst-dependent drug loading of LDI-glycerol polyurethane foams leads to differing controlled release profiles

The purpose of the present study was to develop biodegradable and biocompatible polyurethane foams based on lysine diisocyanate (LDI) and glycerol to be used as drug-delivery systems for the controlled release of 7-tert-butyldimethylsilyl-10-hydroxy-camptothecin (DB-67). The impact of urethane catalysts on cellular proliferation was assessed in an attempt to enhance the biocompatibility of our polyurethane materials. DB-67, a potent camptothecin analog, was then incorporated into LDI-glycerol polyurethane foams with two different amine urethane catalysts: 1,4-diazobicyclo[2.2.2]-octane (DABCO) and 4,4'-(oxydi-2,1-ethane-diyl)bismorpholine (DMDEE). The material morphologies of the polyurethane foams were analyzed via scanning electron microscopy, and DB-67 distribution was assessed by way of fluorescence microscopy. Both foam morphology and drug distribution were found to correlate to the amine catalyst used. Hydrolytic release rates of DB-67 from the polyurethane foams were catalyst dependent and also demonstrated greater drug loads being released at higher temperatures. The foams were capable of delivering therapeutic concentrations of DB-67 in vitro over an 11week test period. Cellular proliferation assays demonstrate that empty LDI-glycerol foams did not significantly alter the growth of malignant human glioma cell lines (P<0.05). DB-67 loaded LDI-glycerol polyurethane foams were found to inhibit cellular proliferation by at least 75% in all the malignant glioma cell lines tested (P<1.0x10(-8)). These results clearly demonstrate the long-term, catalyst-dependent release of DB-67 from LDI-glycerol polyurethane foams, indicating their potential for use in implantable drug-delivery devices.

LDI-glycerol polyurethane implants exhibit controlled release of DB-67 and anti-tumor activity in vitro against malignant gliomas

The purpose of the present study was to develop a biodegradable and biocompatible polyurethane drug delivery system based on lysine diisocyanate (LDI) and glycerol for the controlled release of 7-tert-butyldimethylsilyl-10-hydroxy-camptothecin (DB-67). DB-67 has yet to be implemented in any clinical therapies due to the inability to delivered it in sufficient quantities to impact tumor growth and disease progression. To remedy this, DB-67 was covalently incorporated into our delivery system by way of an organometallic urethane catalyst and was found to be dispersed evenly throughout the LDI-glycerol polyurethane discs. Scanning electron micrographs indicate that the LDI-glycerol discs are uniform and possess a pore distribution typical of the non-solvent casting technique used to prepare them. The release rates of DB-67 from the LDI-glycerol discs were found to vary with both time and temperature and were shown capable of delivering therapeutic concentrations of DB-67 in vitro. Cellular proliferation assays demonstrate that empty LDI-glycerol discs alone do not significantly alter the growth of malignant human glioma cell lines (U87, T98G, LN229 and SG388). DB-67-loaded LDI-glycerol polyurethane discs were found to inhibit cellular proliferation by 50% on average in all the malignant glioma cell lines tested. These results clearly demonstrate the long-term, slow release of DB-67 from LDI-glycerol polyurethane discs and their potential for postoperative intracranial chemotherapy of cancers.

Phosphorylation of beta-actin by protein kinase C-delta in camptothecin analog-induced leukemic cell apoptosis

AIM

This study was conducted to reveal new proteins involved in acute myeloid leukemia (AML) cell apoptosis.

METHODS

Using camptothecin analog NSC606985- induced leukemic U937 cell apoptosis as a model, this study performed a differential proteomic analysis during apoptosis induction. The significantly modulated protein was underwent further investigation in the apoptotic process.

RESULTS

We found that beta-actin protein presented two different spots on the two-dimensional electrophoresis (2-DE) map, which shared similar molecular weight and different pI. Those two spots demonstrated contrary changes (disappeared on the basic-end and increased on the acid-end spot) during apoptosis induction, although the total level of beta-actin kept constant. This observation was further confirmed by immunoblot analysis on 2-DE gel. When NSC606985-treated cell lysate was incubated with alkaline phosphotase, beta-actin on the basic-end spot was restored, indicating increased phosphorylation of beta-actin during NSC606985- induced apoptosis. Moreover, the polymerization of actin also decreased after NSC606985 treatment. The increased beta-actin phosphorylation and decreased actin polymerization was antagonized by pre-treatment of rottlerin, a specific protein kinase C-delta (PKC delta) inhibitor.

CONCLUSION

All these results indicate that beta-actin was phosphorylated during apoptosis induction, which was mediated by activated PKC delta.

Synthesis and molecular structure of ethyl [N-tosyl-(R)-prolyloxy]-2(S)-[4-cyano-8,8-ethylenedioxy-5-oxo-5,6,7,8-tetrahydroindolizin-3-yl] acetate, a key intermediate in the total synthesis of (20S)-camptothecins

The synthesis of optically active [N-tosyl-(R)-prolyloxy]-2(S)-[4-cyano-8,8-ethylenedioxy-5-oxo-5,6,7,8-tetrahydroindolizin-3-yl] acetate (4a), a key intermediate for the total asymmetric synthesis of 20(S)-camptothecin anticancer drugs, is described. Its structure was characterized by 2D-NMR techniques and the absolute configuration was further confirmed for the first time by X-ray crystal structure analysis.

Synthesis and topoisomerase poisoning activity of A-ring and E-ring substituted luotonin A derivatives

A series of A-ring and E-ring analogues of the natural product luotonin A, a known topoisomerase I poison, was evaluated for growth inhibition in human carcinoma and leukemia cell lines. Rational design of structures was based on analogues of the related alkaloid camptothecin, which has been demonstrated to exert cytotoxic effects by the same mechanism of action. When compared to luotonin A, several compounds exhibited an improved topoisomerase I-dependent growth inhibition of a human leukemia cell line.

Antitumor agents. 256. Conjugation of paclitaxel with other antitumor agents: evaluation of novel conjugates as cytotoxic agents

Sixteen different taxoid conjugates were prepared by linking various anticancer compounds, including camptothecin (CPT), epipodophyllotoxin (EP), colchicine (COL), and glycyrrhetinic acid (GA), at the 2'- or 7-position on paclitaxel (TXL, 1) through an ester, imine, amine, or amide bond. Newly synthesized conjugates were evaluated for cytotoxic activity against replication of several human tumor cell lines. Among them, TXL-CPT conjugates, 8-10, were more potent than TXL itself against the human prostate carcinoma cell line PC-3 (ED(50)=14.8, 3.1, 19.4nM compared with 55.5nM), and conjugate 10 was also 8-fold more active than TXL against the LN-CAP prostate cancer cell line. These compounds also possessed anti-angiogenesis ability as well as lower inhibitory effects against a normal cell line (MRC-5). Thus, conjugates 8-10 are possible antitumor drug candidates, particularly for prostate cancer.

Microwave expedited synthesis of 5-aminocamptothecin analogs: Inhibitors of hypoxia inducible factor HIF-1α

A series of 5-aminosubstituted camptothecin analogs were prepared from the corresponding 5-hydroxycamptothecin using microwave irradiation. The analogs were assayed for ability to inhibit the action of hypoxia inducible factors (HIF-1alpha and HIF-2alpha). The 5-fluoroethyl analog showed potent inhibitory activity and is now the focus of ongoing pathway analysis and potential as an antiproliferative agent.