Albumin-Binding Prodrugs of Camptothecin and Doxorubicin with an Ala-Leu-Ala-Leu-Linker That Are Cleaved by Cathepsin B: Synthesis and Antitumor Efficacy

We have recently validated a macromolecular prodrug strategy for improved cancer chemotherapy based on two features: (a) rapid and selective binding of thiol-reactive prodrugs to the cysteine-34 position of endogenous albumin and (b) acid-sensitive promoted or enzymatic release of the drug at the tumor site [Kratz, F., Warnecke, A., Scheuemann, K., Stockmar, C., Schwab, J., Lazar, P., Druckes, P., Esser, N., Drevs, J., Rognan, D., Bissantz, C., Hinderling, C., Folkers, G., Fichtner, I., and Unger, C. (2002) J. Med. Chem. 45, 5523-33]. In the present work, we developed water-soluble camptothecin (CPT) and doxorubicin (DOXO) prodrugs that incorporate the peptide linker Ala-Leu-Ala-Leu that serves as a substrate for the tumor-associated protease, cathepsin B, which is overexpressed in several solid tumors. Consequently, two albumin-binding prodrugs were synthesized [EMC-Arg-Arg-Ala-Leu-Ala-Leu-Ala-CPT (1) and EMC-Arg-Arg-Ala-Leu-Ala-Leu-DOXO (2) (EMC = 6-maleimidocaproic acid)]. Both prodrugs exhibited excellent water-solubility and bound rapidly and selectively to the cysteine-34 position of endogenous albumin. Further in vitro studies showed that the albumin-bound form of the prodrugs was cleaved specifically by cathepsin B as well as in human tumor homogenates. Major cleavage products were CPT-peptide derivatives and CPT for the CPT prodrug and H-Leu-Ala-Leu-DOXO, H-Leu-DOXO, and DOXO for the doxorubicin prodrug. In vivo, 1 was superior to free camptothecin in an HT-29 human colon xenograft model; the antitumor efficacy of prodrug 2 was comparable to that of free doxorubicin in the M-3366 mamma carcinoma xenograft model at equimolar doses.

Highly Efficient and Mild Cascade Reactions Triggered by Bis(triphenyl)oxodiphosphonium Trifluoromethanesulfonate and a Concise Total Synthesis of Camptothecin

A mild and efficient cascade methodology is reported to construct variously substituted indolizino[1,2-b]quinolin-9(11H)-ones. Efficiently triggered by bis(triphenyl)oxodiphosphonium trifluoromethanesulfonate under mild conditions, this cascade achieved significant enhancements in chemical yields. Utilizing this highly efficient domino reaction followed by a Sharpless dihydroxylation, an eight-step total synthesis of camptothecin was accomplished from a known pyridine derivative in direct fashion with an overall yield of 47% and 95% ee.

10-Hydroxycamptothecin loaded nanoparticles: preparation and antitumor activity in mice

10-Hydroxycamptothecin (HCPT) loaded nanoparticles made from poly(caprolactone-co-lactide)-b-PEG-b-poly(caprolactone-co-lactide) (PCLLA-PEG-PCLLA) block copolymer, were prepared by a novel two-step nanoprecipitation method using an interior-chemistry strategy. The satisfactory drug loading content (>13%) as well as high encapsulation efficiency (>85%) was achieved. Cytotoxicity test indicated that the HCPT-loaded nanoparticles had enhanced in vitro cytotoxicity compared to free drug. Progressively, in vivo antitumor activity and HCPT biodistribution in sarcoma-180 (S-180) bearing mice after intravenous injection of the HCPT-loaded nanoparticles show that HCPT-loaded nanoparticles exhibited superior in vivo antitumor effect and remarkably different biodistribution than the commercially available HCPT injection.

Soluble camptothecin derivatives prepared by click cycloaddition chemistry on functional aliphatic polyesters

Aliphatic polyesters are of interest as biomaterials and drug-delivery vehicles, as their ability to degrade under physiological conditions provides a mechanism for both drug release and clearance of the polymer from the body. Presented here is the synthesis of a polyester-drug graft copolymer conjugate, enabled by click cycloaddition of azide-functionalized camptothecin derivatives with alkyne-functionalized aliphatic polyesters. Further grafting of residual alkyne groups with azide-terminated poly(ethylene oxide) gave a water-soluble polyester-camptothecin conjugate. Control over PEGylation and drug loading, inherent to the graft copolymer design, opens versatile routes to new materials with potential utility in polymer therapeutics.

Development of novel oxygen-independent photosensitizers

We proposed a strategy of photooxidizer-reduction activated alkylator (P-A) hybrid molecule to develop novel oxygen-independent photosensitizers. Two prototypes of such photosensitizers camptothecin-indolequinone (CPT-IQ) and camptothecin-nitrofuryl (CPT-NF) was designed and prepared. A mechanism of photo-induced oxidation and alkylation of 2'-deoxyguanosine by CPT-IQ was investigated. CPT-NF was confirmed to effectively induce DNA cleavage via 365-nm UV irradiation both under normaxia and hypoxia.

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.

Synthesis and antitumor activity of novel 10-substituted camptothecin analogues

In an attempt to improve the antitumor activity and decrease the cytotoxicity of camptothecin, 18 new 10-substituted camptothecin derivatives were prepared. The cytotoxicity in vitro on cancer cell lines and antitumor activity in vivo, and inhibitory properties of topoisomerase I of these derivatives were evaluated. Most of these derivatives possessed lower cytotoxicities than CPT, and the compounds 13, 21, 22, 23, and 24 showed similar topoisomerase I inhibitory activity to CPT. Analogues 13 exhibited the best antitumor activity in vivo among all derivatives we prepared.

Kinetics and mechanisms of activation of alpha-amino acid ester prodrugs of camptothecins

The alpha-amino acid ester prodrugs of the antitumor agent camptothecin and a more potent, lipophilic silatecan analogue, DB-67, have been shown by NMR spectroscopy and quantitative kinetic analyses to undergo quantitative conversion to their pharmacologically active lactones via a nonenzymatic mechanism that at pH 7.4 is favored over direct hydrolysis. The alternate pathway involves the reversible intramolecular nucleophilic amine attack at the camptothecin E-ring carbonyl to generate a lactam (I) followed by a second intramolecular reaction to produce a bicyclic hemiortho ester (I'). The intermediates were isolated and shown to exist in an apparent equilibrium dominated by the hemiortho ester in DMSO using NMR spectroscopy. The conversion of prodrugs of camptothecin or DB-67 containing either alpha-NH(2) or alpha-NHCH(3) and their corresponding hemiortho esters were monitored versus time in aqueous buffer (pH 3.0 and 7.4) at 37 degrees C, and the kinetic data were fit to a model based on the proposed mechanism. The results indicated that while the prodrugs are relatively stable at pH 3, facile lactone release occurs from both the prodrugs and their corresponding hemiortho ester intermediates under physiological conditions (pH 7.4). The glycinate esters and their hemiortho esters were found to be more cytotoxic than the N-methylglycinates or their corresponding hemiortho ester intermediates in vitro using a human breast cancer cell line (MDA-MB-435S), consistent with their more rapid conversion to active lactone. The pH dependence of the nonenzymatic pathway for conversion of these alpha-amino acid ester prodrugs suggests that they may be useful for tumor-targeting via liposomes, as they can be stabilized in an acidic environment in the core of liposomes and readily convert to the active lactone following their intratumoral release.

Practical Formal Total Syntheses of the Homocamptothecin Derivative and Anticancer Agent Diflomotecan via Asymmetric Acetate Aldol Additions to Pyridine Ketone Substrates

Two practical, efficient, and scalable asymmetric routes to DE ring fragment 7, a key building block in the synthesis of the homocamptothecin derivative diflomotecan 4, are described. The "acetal route" starts from 2-chloro-4-cyanopyridine 8 and represents an enantioselective and optimized modification of the original racemic discovery chemistry synthesis. The inefficient optical resolution procedure was replaced by an efficient asymmetric acetate aldol addition (dr 87:13) to a ketone substrate as the key step generating the (R)-configured quaternary stereocenter with high stereoselectivity. 7 was finally obtained in 8.9% overall yield (er 99.95:0.05) over nine steps, avoiding chromatographic purifications and comparing favorably with the initial procedure. In the related "amide route" starting from 2-chloroisonicotinic acid 41, a secondary amide directing group was used to facilitate the ortho lithiation of the pyridine 3-position. The key step of this protocol again consists of a practical asymmetric acetate aldol addition (dr = 87:13). The DE ring building block 7 was thus obtained in 11.1% overall yield (er > 99.95:0.05) over nine steps requiring only one chromatographic purification.

Synthesis and activity of a folate peptide camptothecin prodrug

A folate receptor targeted camptothecin prodrug was synthesized using a hydrophilic peptide spacer linked to folate via a releasable disulfide carbonate linker. The conjugate was found to possess high affinity for folate receptor-expressing cells and inhibited cell proliferation in human KB cells with an IC(50) of 10nM. Activity of the prodrug was completely blocked by excess folic acid, demonstrating receptor-mediated uptake.

Synthesis and Cytotoxic Activity of Polyamine Analogues of Camptothecin

A number of derivatives of camptothecin with a polyamine chain linked to position 7 of camptothecin via an amino, imino, or oxyiminomethyl group were synthesized and tested for their biological activity. All compounds showed marked growth inhibitory activity against the H460 human lung carcinoma cell line. In particular, the iminomethyl derivatives where the amino groups of the chain were protected with Boc groups exhibited a high potency, with IC50 values of approximately 10(-8) M. The pattern of DNA cleavage in vitro and the persistence of the cleavable ternary complex drug-DNA-topoisomerase I observed with polyamine conjugates containing free amino groups support a contribution of specific drug interaction with DNA as a determinant of activity. Modeling of compound 7c in the complex with topoisomerase 1 and DNA is consistent with this hypothesis. The lack of a specific correlation between stabilization of the cleavable complex and growth inhibition likely reflects multiple factors including the cellular pharmacokinetic behavior related to the variable lipophilicity of the conjugate, and the nature and linkage of the polyamine moiety.

Synthesis and Biological Evaluation of 10,11-Methylenedioxy-14-azacamptothecin

[reaction: see text] 10,11-Methylenedioxy-14-azacamptothecin, a potent analogue of the antitumor agent camptothecin (CPT), has been prepared via a key condensation between AB and DE ring precursors. The biological testing of this compound validated a strategy for modulation of the off-rate of camptothecin analogues from the topoisomerase-DNA-CPT ternary complex via structural modification.

Review Camptothecin: Current Perspectives

The review provides a detailed discussion of recent advances in the medicinal chemistry of camptothecin, a potent antitumor agent that targets topoisomerase I. Thousands of CPT derivatives have been synthesized. Two of them, Topotecan and Irinotecan, are commercially approved for use in clinic as antitumor agents while more are still in clinic trials. This review summarizes the current status of the modern synthetic approaches to CPT, the mechanism of action of CPT, the structure-activity relationship(SAR), a number of novel CPT analogs and their biologic activity. There is a systematic evaluation of A-, B- and E-ring- modified camptothecins reported recently.

Synthesis and biological assays of E-ring analogs of camptothecin and homocamptothecin

Analogs of the anti-tumor agent camptothecin with both closed E-rings (lactone and ether) and open E-rings (reduced acid, hydrazide, and protected Weinreb amide) have been prepared and tested in topoisomerase and cellular assays. The results provide insights into the structural features of the camptothecin E-ring that affect biological activity.

Novel polymeric prodrug with multivalent components for cancer therapy

We designed, synthesized, and evaluated in vitro and in vivo a novel targeted anticancer polymeric prodrug containing multiple copies of tumor targeting moiety [synthetic luteinizing hormone-releasing hormone (LHRH) peptide, analog of LHRH] and anticancer drug (camptothecin). One, two, or three molecules of the targeting peptide and anticancer drug were covalently conjugated with bis(2-carboxyethyl) polyethylene glycol polymer using citric acid as a multivalent spacer. We showed that LHRH peptide was bound to extracellular receptors and localized in plasma membrane of cancer cells. The designed tumor-targeted prodrug increased the solubility of anticancer drug and offered cytoplasmic and/or nuclear delivery of drug to cancer cells expressing LHRH receptors. The multicomponent prodrug containing three copies of the targeting peptide and drug was almost 100 times more cytotoxic and substantially had enhanced antitumor activity compared with the analogous nontargeted prodrug and prodrugs containing one or two copies of active components.

Novel, efficient total synthesis of natural 20(S)-camptothecin

Enantiopure 20(S)-camptothecin has been prepared from a known hydroxypyridone through a novel approach that involves a Claisen rearrangement, an asymmetric nucleophilic ethylation, a Heck coupling and a Friedländer condensation as the key transformations.

Transport Mechanism-Based Drug Molecular Design: Novel Camptothecin Analogues to Circumvent ABCG2-associated Drug Resistance of Human Tumor Cells

Acquired and intrinsic drug resistance in cancer is the major obstacle to long-term, sustained patient response to chemotherapy. Irinotecan (CPT-11) is a widely-used potent antitumor drug that inhibits mammalian DNA topoisomerase I (Topo I). However, overexpression of ABCG2 (BCRP/MXR/ABCP) reportedly confers cancer cells resistance to SN-38, the active form of CPT-11. To circumvent the ABCG2-associated drug resistance, we have synthesized and characterized a total of fourteen new camptothecin (CPT) analogues with respect to both the inhibition of Topo I and the substrate specificity of ABCG2. While the lactone E ring is a prerequisite for anticancer activity, modifications of the A or B rings do not significantly affect Topo I inhibition activity. In this context, we have synthesized new CPT analogues with different substitutions at positions 10 or 11 of the A ring. All of the tested CPT analogues strongly inhibited the Topo I activity in a cell-free system. Accordingly, we have examined ATP-dependent transport of those CPT analogues by using plasma membrane vesicles prepared from ABCG2-overexpressing cells. Based on the substrate specificity of ABCG2 thus evaluated, it is strongly suggested that CPT analogues with a hydroxyl group at position 10 or 11 of the A ring are good substrates for ABCG2 and therefore effectively extruded from cancer cells. Thus, hydrogen bond formation is considered to be involved in substrate recognition and/or transport processes of ABCG2. The present study provides a practical approach to discover new CPT-based drugs for the chemotherapy of drug-resistant human cancer.

Practical formal total synthesis of (rac)- and (S)-camptothecin

A practical, efficient and scalable formal total synthesis of (rac)- and (S)-camptothecin is described, which proceeds via the known DE ring building blocks 19 and (S)-19, respectively. The racemic synthesis starts from diethyl oxalate and uses straightforward carbonyl chemistry in order to generate the pyridone ring system. 19 was formed in 8.4% overall yield over 9 linear steps avoiding any chromatographic purification. The asymmetric version of this approach encompassed a diastereoselective Grignard addition to the enantiomerically pure alpha-ketoester 30 in order to generate the (S)-configured quaternary stereocenter. The auxiliary could be recycled in high yield and was successfully reused multiple times. The final steps paralleled the racemic approach. (S)-19 was thus prepared in 9.4% overall yield (er = 95 : 5) over 10 steps.

[Synthesis of camptothecin glycosides by phase transfer-catalysis and its inhibitory activity against topo I]

AIM

To find new anticancer drug based on the structure of 10-hydroxy camptothecin.

METHODS

Six camptothecin glycosides (7-12) were synthesized by phase transfer catalysis. The structures of all compounds synthesized were determined by 1H NMR, IR and MS. Their antitumor activity was evaluated on cancer cells in vitro, and inhibitory activity against Topo I was evaluated by molecular biologic method.

RESULTS AND CONCLUSION

The result indicated that the yield of camptothecin glycosides by phase transfer catalysis is much higher than by the method from literature, camptothecin glycosides have much lower cytotoxicities on cancer cell in vitro, but have better inhibitory activity of topo I.

[Synthesis and spectral characterization of 10-hydroxycamptothecin]

10-hydroxycamptothecin was synthesized from camptothecin through oxidation and photo-activation. Grouping and optimizing the reaction conditions were as follows: in the oxidation condition: the reaction time 4 h, the reaction temperature 75 degrees C, the amount of H2O2 48 mL(w(H2O2) = 30%), the amount of HAc 350 mL(based on 0.01 mol CPT); in the photochemical reaction: the solvents V(1,4-dioxane): V(acetonitrile): V(H2O) = 6:2:1, bronsted acid catalyst 98% H2SO4. Purified by silica gel column, the total yield of HCPT is 49.9%, with 99.5% purity, m.p. 272-273 degrees C. The molecular structure of the title compound has been characterized by elemental analysis (EA), mass spectroscopy (MS), infrared spectrscopy(IR), and ID and 2D nuclear magnetic resonance spectrometry (NMR), and the main infrared absorption peaks and nuclear magnetic spectral bands of this conpound were assigned. The mass spectral fragmentations of the product's important fragment ions were elucidated. The result provides useful information for preparing the new derivatives of camptothecin.

Total and semisynthesis and in vitro studies of both enantiomers of 20-fluorocamptothecin

Both enantiomers of 20-fluorocamptothecin and the racemate have been prepared by total synthesis. The (R)-enantiomer is essentially inactive in a topoisomerase-I/DNA assay, while the (S)-enantiomer is much less active than (20S)-camptothecin. The lactone ring of 20-fluorocamptothecin hydrolyzes more rapidly than that of camptothecin in PBS. The results provide insight into the role of the 20-hydroxy group in the binding of camptothecin to topoisomerase-I and DNA.

Novel fluoro-substituted camptothecins: in vivo antitumor activity, reduced gastrointestinal toxicity and pharmacokinetic characterization

PURPOSE

The novel fluoro-substituted camptothecin analog, BMS-286309, and its prodrug, BMS-422461, were evaluated for their pharmacologic, toxicologic, metabolic and pharmacokinetic developmental potential.

METHODS

In vitro and in vivo assays were used to assess the compounds for topoisomerase I activity, antitumor activity, gastrointestinal (GI) toxicity, and pharmacokinetic parameters.

RESULTS

BMS-286309-induced topoisomerase I-mediated DNA breaks in vitro and was similar in potency to camptothecin. Both BMS-286309 and -422461 were comparable to irinotecan regarding preclinical antitumor activity assessed in mice bearing distal site murine and human tumors. BMS-422461 was also found to be orally active. Both analogs were >100-fold more potent in vivo than irinotecan and both were superior to irinotecan with respect to toxicological assessment of GI injury in mice. The generation of parent compound from BMS-422461 was qualitatively similar in mouse, rat and human blood and liver S9 fractions. The percentage of BMS-286309 remaining as the active lactone form at equilibrium was comparable in mouse and human plasma. The pharmacokinetic profile in rat blood demonstrated that BMS-422461 was rapidly cleaved to BMS-286309.

CONCLUSIONS

The favorable in vivo metabolic activation of BMS-422461, and the pharmacokinetic characteristics of BMS-286309, suggest that the good efficacy of BMS-422461 is derived from robust in vivo release of BMS-286309 in rodents and the likelihood that this biotransformation will be preserved in humans. The comparable antitumor activity of BMS-422461 to irinotecan, as well as reduced preclinical GI toxicity, make this novel camptothecin analog attractive for clinical development.

Synthesis and antitumor activity of the hexacyclic camptothecin derivatives

A series of hexacyclic camptothecin derivatives were synthesized to test for antitumor activity as topoisomerase I inhibitor. The strategy of synthesis was used for the formation of additional furan and dihydrofuran rings fused with 9- and 10-positions of camptothecin. All of the hexacyclic camptothecins were assayed for cytotoxicity against four human tumor cell lines, HL60, BEL-7402, HCT-116, and HeLa, and showed very impressive cytotoxicity activity in vitro. Enzyme activity of the hexacyclic camptothecins was evaluated, being equal or superior to that of SN-38. The stability of four compounds was assessed in human plasma. Two of these compounds were chosen to test for antitumor activity in vivo against Sarcoma-180. The results suggested that additional furan and dihydrofuran rings could improve the antitumor activity in vitro and vivo, though the stability of the lactone ring did not increase.

Synthesis and evaluation of a DHA and 10-hydroxycamptothecin conjugate

We have synthesized a conjugate of cis-4,7,10,13,16,19-docosahexenoic acid (DHA) and 10-hydroxycamptothecin (HCPT), DHA-HCPT. The antitumor activity of DHA-HCPT was evaluated in vitro against L1210 leukemia cells and in experimental animal tumor models including L1210 leukemia, Lewis lung carcinoma, and colon 38 adenocarcinoma. DHA-HCPT showed a greatly improved antitumor efficacy compared to HCPT.

Radiosynthesis of carbon-11-labeled camptothecin derivatives as potential positron emission tomography tracers for imaging of topoisomerase I in cancers

Four carbon-11-labeled camptothecin derivatives, 9-[11C]methoxy-20(S)-camptothecin ([11C]5), 10-[11C]methoxy-20(S)-camptothecin ([11C]7), 9-nitro-10-[11C]methoxy-20(S)-camptothecin ([11C]9), and 9-[([11C]trimethylamino)methyl]-10-hydroxy-20(S)-camptothecin ([11C]11), have been synthesized as potential positron emission tomography tracers for imaging of topoisomerase I in cancers.