Chemical modification of an antitumor alkaloid camptothecin: synthesis and antitumor activity of 7-C-substituted camptothecins

Identification of 7-aminourea or 7-aminothiourea derivatives of camptothecin as selective topoisomerase I inhibitors with anti-colorectal cancer activities

Colorectal cancer (CRC) remains one of the most prevalent malignant tumors of the digestive system, yet the availability of safe and effective chemotherapeutic agents for clinical use remains limited. Camptothecin (CPT) and its derivatives, though approved for cancer treatment, have encountered significant challenges in clinical application due to their low bioavailability and high systemic toxicity. Strategic modification at the 7-position of CPT enables the development of novel CPT derivatives with high activity. In the present study, a series of compounds incorporating aminoureas, amino thioureas, and acylamino thioureas as substituents at the 7-position were screened. These compounds were subsequently evaluated for their cytotoxicity against the human gastric cancer (GC) cell line AGS and the CRC cell line HCT116. Two derivatives, XSJ05 (IC50 = 0.006 ± 0.003 μM) and XSJ07 (IC50 = 0.013 ± 0.003 μM), exhibited remarkably effective anti-CRC activity, being better than TPT. In addition, they have a better safety profile. In vitro mechanistic studies revealed that XSJ05 and XSJ07 exerted their inhibitory effects on CRC cell proliferation by suppressing the activity of topoisomerase I (Topo I). This suppression triggers DNA double-strand breaks, leads to DNA damage and subsequently causes CRC cells to arrest in the G2/M phase. Ultimately, the cells undergo apoptosis. Collectively, these findings indicate that XSJ05 and XSJ07 possess superior activity coupled with favorable safety profiles, suggesting their potential as lead compounds for the development of CRC therapeutics.

Development of an enzyme‑linked immunosorbent assay for camptothecin

The use of camptothecin and its analogues has increased in clinical settings and in agriculture. Therefore, camptothecins and their derivatives, metabolites and degradation products are frequently found in the environment. Therefore, it is important to develop an ELISA for the quantification of camptothecins in human plasma, plants, animal tissues and other matrices. The present study developed a novel competitive indirect ELISA for camptothecin using a monoclonal antibody (MAb). In total, two haptens and various carrier proteins were tested to select the most suitable immunogen for the production of MAbs against camptothecin. Hapten 1 conjugated with keyhole limpet hemocyanin was selected for the preparation of MAb 5A3, and was used to establish a competitive indirect ELISA for camptothecin. A total of three derivatives of camptothecin used in clinical practice were examined. Topotecan showed an IC₅₀ value of 0.68 µg/ml with a detection limit of 0.19 µg/ml, belotecan showed an IC₅₀ value of 0.87 µg/ml with a detection limit of 0.22 µg/ml and irinotecan showed an IC₅₀ value of 2.85 µg/ml with a detection limit of 0.47 µg/ml. The cross-reactivity results suggested that the assay developed in the present study possessed a high sensitivity to camptothecin. Therefore, this immunoassay technique may be suitable for monitoring the levels of camptothecin in compound analysis, clinical applications, and analyses of food and environmental samples.

Design and synthesis of novel 7-[(N-substituted-thioureidopiperazinyl)-methyl]-camptothecin derivatives as potential cytotoxic agents

As part of continuing our research on diverse C-7 derivatives of camptothecin (CPT), 16 CPT derivatives bearing piperazinyl-thiourea chemical scaffold and different substituent groups have been designed, synthesized and evaluated in vitro for cytotoxicity against five tumor cell lines (A-549, MDA-MB-231, MCF-7, KB and KBvin). As a result, all the synthesized compounds showed promising in vitro cytotoxic activity against the five tumor cell lines tested, and were more potent than irinotecan. Importantly, compounds 13 g (IC₅₀ = 0.514 μM) and 13o (IC₅₀ = 0.275 μM) possessed similar or better antiproliferative activity against the multidrug-resistant (MDR) KBvin subline than that of topotecan (IC₅₀ = 0.511 μM) and merit further development as anticancer candidates for clinical trail. With these results in hand, we have a reason to conclude that incorporating piperazinyl-thiourea motifs into position-7 of camptothecin confers well cytotoxic activity against cancer cell lines, probably resulting in new anticancer drugs.

Design, synthesis, and cytotoxic activity of novel 7-substituted camptothecin derivatives incorporating piperazinyl-sulfonylamidine moieties

In our continuing search for camptothecin (CPT)-derived antitumor drugs, novel 7-substituted CPT derivatives incorporating piperazinyl-sulfonylamidine moieties were designed, synthesized and evaluated for cytotoxicity against five tumor cell lines (A-549, MDA-MB-231, MCF-7, KB, and KB-VIN). All of the derivatives showed promising in vitro cytotoxic activity against the tested tumor cell lines, and were more potent than irinotecan. Remarkably, most of the compounds exhibited comparable cytotoxicity against the multidrug-resistant (MDR) KB-VIN and parental KB tumor cell lines, while irinotecan lost activity completely against KB-VIN. Especially, compounds 13r and 13p (IC₅₀ 0.38 and 0.85μM, respectively) displayed the greatest cytotoxicity against the MDR KB-VIN cell line and merit further development into preclinical and clinical drug candidates for treating cancer, including MDR phenotype.

Design, synthesis and potent cytotoxic activity of novel 7-(N-[(substituted-sulfonyl)piperazinyl]-methyl)-camptothecin derivatives

In an effort to discover potent camptothecin-derived antitumor agents, novel camptothecin analogues with sulfonylpiperazinyl motifs at position-7 were designed and synthesized. They were evaluated for in vitro cytotoxicity with the sulforhodamine-B (SRB) method in five types of human tumor cell lines, A-549, MDA-MB-231, KB, KB-VIN and MCF-7. With IC50 values in the low μM to nM level, most of the new analogues showed greater cytotoxicity activity than the reference compounds irinotecan and topotecan. Furthermore, compounds 12l (IC50, 1.2nM) and 12k (IC50, 20.2nM) displayed the highest cytotoxicity against the multidrug-resistant (MDR) KB-VIN cell line and merit further development as preclinical drug candidates for treating cancer, including MDR phenotype. Our study suggested that integration of sulfonylpiperazinyl motifs into position-7 of camptothecin is an effective strategy for discovering new potent cytotoxic camptothecin derivatives.

Synthesis, biological activities, and quantitative structure-activity relationship (QSAR) study of novel camptothecin analogues

In continuation of our program aimed at the development of natural product-based pesticidal agents, three series of novel camptothecin derivatives were designed, synthesized, and evaluated for their biological activities against T. Cinnabarinus, B. brassicae, and B. xylophilus. All of the derivatives showed good-to-excellent activity against three insect species tested, with LC50 values ranging from 0.00761 to 0.35496 mmol/L. Remarkably, all of the compounds were more potent than CPT against T. Cinnabarinus, and compounds 4d and 4c displayed superior activity (LC50 0.00761 mmol/L and 0.00942 mmol/L, respectively) compared with CPT (LC50 0.19719 mmol/L) against T. Cinnabarinus. Based on the observed bioactivities, preliminary structure-activity relationship (SAR) correlations were also discussed. Furthermore, a three-dimensional quantitative structure-activity relationship (3D-QSAR) model using comparative molecular field analysis (CoMFA) was built. The model gave statistically significant results with the cross-validated q2 values of 0.580 and correlation coefficient r2 of 0.991 and  of 0.993. The QSAR analysis indicated that the size of the substituents play an important in the activity of 7-modified camptothecin derivatives. These findings will pave the way for further design, structural optimization, and development of camptothecin-derived compounds as pesticidal agents.

FL118, a novel camptothecin derivative, is insensitive to ABCG2 expression and shows improved efficacy in comparison with irinotecan in colon and lung cancer models with ABCG2-induced resistance

Background

Irinotecan is a camptothecin analogue currently used in clinical practice to treat advanced colorectal cancer. However, acquired resistance mediated by the drug efflux pump ABCG2 is a recognized problem. We reported on a novel camptothecin analogue, FL118, which shows anticancer activity superior to irinotecan. In this study, we sought to investigate the potency of FL118 versus irinotecan or its active metabolite, SN-38, in both in vitro and in vivo models of human cancer with high ABCG2 activity. We also sought to assess the potency and ABCG2 affinity of several FL118 analogues with B-ring substitutions.

Methods

Colon and lung cancer cells with and without ABCG2 overexpression were treated with FL118 in the presence and absence of Ko143, an ABCG2-selective inhibitor, or alternatively by genetically modulating ABCG2 expression. Using two distinct in vivo human tumor animal models, we further assessed whether FL118 could extend time to progression in comparison with irinotecan. Lastly, we investigated a series of FL118 analogues with B-ring substitutions for ABCG2 sensitivity.

Results

Both pharmacological inhibition and genetic modulation of ABCG2 demonstrated that, in contrast to SN-38, FL118 was able to bypass ABCG2-mediated drug resistance. FL118 also extended time to progression in both in vivo models by more than 50% compared with irinotecan. Lastly, we observed that FL118 analogues with polar substitutions had higher affinity for ABCG2, suggesting that the nonpolar nature of FL118 plays a role in bypassing ABCG2-mediated resistance.

Conclusions

Our results suggest that in contrast to SN-38 and topotecan, FL118 is a poor substrate for ABCG2 and can effectively overcome ABCG2-mediated drug resistance. Our findings expand the uniqueness of FL118 and support continued development of FL118 as an attractive therapeutic option for patients with drug-refractory cancers resulting from high expression of ABCG2.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-015-0362-9) contains supplementary material, which is available to authorized users.

Design and synthesis of new 7-(N-substituted-methyl)-camptothecin derivatives as potent cytotoxic agents

A series of novel 7-(N-substituted-methyl)-camptothecin derivatives was designed, synthesized, and evaluated for in vitro cytotoxicity against four human tumor cell lines, A-549, MDA-MB-231, KB, and KBvin. All of the derivatives showed promising in vitro cytotoxic activity against the tested tumor cell lines, with IC50 values ranging from 0.0023 to 1.11 μM, and were as or more potent than topotecan. Compounds 9d, 9e, and 9r exhibited the highest antiproliferative activity among all prepared derivatives. Furthermore, all of the compounds were more potent than paclitaxel against the multidrug-resistant (MDR) KBvin subline. With a concise efficient synthesis and potent cytotoxic profiles, especially significant activity towards KBvin, compounds 9d, 9e, and 9r merit further development as a new generation of camptothecin-derived anticancer clinical trial candidates.

Semisynthesis, cytotoxic activity, and oral availability of new lipophilic 9-substituted camptothecin derivatives

Despite that 9-substituted camptothecins are promising candidates in cancer therapy, the limited accessibility to this position has reduced the studies of these derivatives to a few standard modifications. We report herein a novel semisynthetic route based on the Tscherniac-Einhorn reaction to synthesize new lipophilic camptothecin derivatives with amidomethyl and imidomethyl substitutions in position 9. Compounds were evaluated for their antiproliferative activity, topoisomerase I inhibition, and oral availability. Preliminary data demonstrated that bulky imidomethyl modification is an appropriate lipophilic substitution for an effective oral administration relative to topotecan. In addition, this general procedure paves the way for obtaining new camptothecin derivatives.

Synthesis and biological evaluation of 7-alkenyl homocamptothecins as potent topoisomerase I inhibitors

Homocamptothecin (hCPT) is a camptothecin (CPT) derivative with a seven-membered β-hydroxylactone E ring, which shows higher lactone stability and improves topoisomerase I (Topo I) inhibition activity. In an attempt to improve the antitumor activity of homocamptothecins, a series of 7-alkenyl-homocamptothecin derivatives was designed and synthesized based on a semisynthetic route starting from CPT. Most of the synthesized compounds exhibit higher cytotoxic activities on the A-549 tumor cell line than topotecan (TPT). Some compounds such as 2a and 2o show a broad in vitro antitumor spectrum and exhibit superior Topo I-inhibition activity.

Electron paramagnetic resonance (EPR) study of spin-labeled camptothecin derivatives: a different look of the ternary complex

Camptothecin (CPT) derivatives are clinically effective poisons of DNA topoisomerase I (Top1) able to form a ternary complex with the Top1-DNA complex. The aim of this investigation was to examine the dynamic aspects of the ternary complex formation by means of site-directed spin labeling electron paramagnetic resonance (SDSL-EPR). Two semisynthetic CPT derivatives bearing the paramagnetic moiety were synthesized, and their biological activity was tested. A 22-mer DNA oligonucleotide sequence with high affinity cleavage site for Top1 was also synthesized. EPR experiments were carried out on modified CPT in the presence of DNA, of Top1, or of both. In the last case, a slow motion component in the EPR signal appeared, indicating the formation of the ternary complex. Deconvolution of the EPR spectrum allowed to obtain the relative drug amounts in the complex. It was also possible to demonstrate that the residence time of CPT "trapped" in the ternary complex is longer than hundreds of microseconds.

Synthesis and Pharmacological Evaluation of Novel Homocamptothecin-Dihydropyridine Derivative Conjugates as Potent Topoisomerase I Inhibitors

Homocamptothecins (hCPT) represent a new generation of antitumour agents targeting DNA topoisomerase I. The expanded seven-membered lactone E-ring that characterizes hCPT enhances the plasma stability of the drug and reinforces the inhibition of topoisomerase I (Topo I) compared with conventional six-membered CPT. In an attempt to improve the antitumour activity of hCP, a series of novel hCPT derivatives conjugating with dihydropyridine derivates were designed and synthesized based on a synthetic route that couples 7-formylhomocamptothecin with different dihydropyridine derivates. Most of the synthesized compounds exhibited good cytotoxic activity on tumour cell line A549, MDA-MB-435, and HCT116. Furthermore, this class of compounds showed superior Topo I inhibition activity comparable to or higher than CPT.

Antitumor Activities and Structure - Activity Relationship of Phosphotriester Derivatives of Homocamptothecin Based on a Semisynthetic Route

Based on a new semisynthetic route, a series of phosphotriester 7-alkyl-homocamptothecin derivatives are designed and synthesized. Cytotoxic activity assays show that compounds 12a and 14c with a methyl in position C7 of the homocamptothecin are more potent than the other derivatives and the positive drug irinotecan against A-549, MCF-7, and LOVO cell lines. Moreover, compound 14c shows potent tumour inhibitory activity in a Colo205 xenograft model.

The anti-HIV actions of 7- and 10-substituted camptothecins

Camptothecin (CPT), a traditional anti-tumor drug, has been shown to possess anti-HIV-1 activity. To increase the antiviral potency, the anti-HIV activities of two CPT derivatives, 10-hydroxy-CPT and 7-hydroxymethyl-CPT, were evaluated in vitro. The therapy index (TI) of CPT, 10-hydroxy-CPT and 7-hydroxymethyl-CPT against HIV-1_IIIB in C8166 were 24.2, 4.2 and 198.1, and against clinical isolated strain HIV-1_KM018 in PBMC were 10.3, 3.5 and 66.0, respectively. While the TI of CPT, 10-hydroxy-CPT and 7-hydroxymethyl-CPT against HIV-2_CBL-20 were 34.5, 10.7 and 317.0, respectively, and the TI of the three compounds against HIV-2_ROD showed the similar values. However, when the antiviral mechanisms were considered, we found there was no inhibition of 7-hydroxymethyl-CPT on viral cell-to-cell transmission, and was no inhibition on reverse transcriptase, protease or integrase in cell-free systems. 7-Hydroxymethyl-CPT showed no selective killing of chronically infected cells after 3 days of incubation. In conclusion, 7-hydroxymethyl-CPT showed more potent anti-HIV activity, while 10-hydroxy-CPT had less efficient activity, compared with the parent CPT. Though the antiviral mechanisms remain to be further elucidated; the modification of -OH residues at C-7 of CPT could enhance the antiviral activity, while of -OH residues at C-10 of CPT had decreased the antiviral activity, which provides the preliminary modification strategy for anti-viral activities enhancement of this compound.

A phase II trial of irinotecan in patients with previously untreated advanced esophageal and gastric adenocarcinoma

Chemotherapy options for esophagogastric adenocarcinoma remain limited. Irinotecan has demonstrated broad activity in a variety of epithelial malignancies. Forty-six patients with previously untreated, measurable, unresectable, or metastatic esophagogastric adenocarcinoma were enrolled. Patients received irinotecan (125 mg/m2 intravenously over 90 min weekly) for 4 consecutive weeks followed by a 2-week rest. Forty-three patients received at least one treatment and were evaluable for response and toxicity. One complete and five partial responses were observed, for an overall response rate of 14% (95% CI, 4-24%). Median survival for all 43 patients was 6.4 months (95% CI, 4.6-8.2 months). Grade 3 to 4 toxicity included 10 patients (23%) with neutropenia, 13 patients (30%) with late diarrhea, 6 patients (14%) with vomiting, and 6 patients (14%) with fatigue. We conclude that although single-agent irinotecan is an active agent for esophagogastric adenocarcinoma, the schedule utilized in this trial is associated with moderate toxicity. When used as a single-agent, a tri-weekly schedule may be preferable for this patient population.

Camptothecin analogs with enhanced activity against human breast cancer cells. I. Correlation of potency with lipophilicity and persistence in the cleavage complex

The effect of 7-alkyl substitutions on growth inhibition in seven Camptothecin (CPT) ring systems with various groups at the ten position was evaluated in three human breast cancer cell lines that model (1) hormone-sensitive (MCF-7/wt), (2) hormone insensitive (MDA-MB-231), or (3) alkylator-resistant (MCF-7/4-hc) forms of disease. To assess the impact of persistence of cleavage complexes on antiproliferative activity, a post-exposure recovery period in drug-free medium was incorporated into the growth inhibition assay. This modification produced on average a twofold reduction in the growth inhibition endpoint (the IC50), suggesting a greater apoptotic response. The results further revealed a three log range in potency from a mean IC50 of 2 nM (7-butyl-10,11-methylenedioxy-CPT) to 2.5 microM (7-bromomethyl-10-hydryoxy-CPT). Increasing 7-alkyl chain length in six of the ten-substituted CPTs enhanced potency, which was directly correlated with persistence of topoisomerase I-induced DNA cleavage complexes in 10-hydroxy, 10-methoxy, and 10,11-methylenedioxy substituted CPTs. Modeling of the binding mode of 7-butyl-10-amino-CPT revealed a direct hydrogen bond contact for the 10-amino to the side chain of Glu-356 of Core Subdomain I of top1 in addition to known contacts found for other camptothecins. More important, residues 350-356 and 425-431 of Core Subdomain I may provide induced fit stabilization to the lipophilic alkyl moiety at the seven position.

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.

Camptothecin analogs with bulky, hydrophobic substituents at the 7-position via a Grignard reaction

By developing a new synthetic procedure for introduction of side chains onto the camptothecin ring system, we were able to achieve the preparation of a number of analogs bearing bulky, hydrophobic groups directly attached to the 7-position. These include 7-tert-butylcamptothecin, 7-benzylcamptothecin and the corresponding 10,11-methylenedioxycamptothecins. This method involves the reaction of an appropriate orthoaminobenzonitrile with various Grignard reagents to give the corresponding orthoaminoketones. Friedlander condensation of the latter with the key tricyclic ketone leads to 7-substituted camptothecin analogs. We report the activity of these compounds as topoisomerase I poisons and their ability to inhibit growth of selected tumor cell lines.

Synthesis and cytotoxic activity of substituted 7-aryliminomethyl derivatives of camptothecin

A series of imines derived from camptothecin-7-aldehyde (CPT-CHO) and aromatic amines were synthesised and tested for their cytotoxicity against tumour cell line H460, that expresses a high level of topoisomerase I. In general ortho-substituted compounds showed higher cytotoxic potency than the corresponding para-substituted imines. This effect was dependent on the nature of the substituent. Structure-activity relationships were studied by calculation of docking energy with a model of the ternary complex camptothecin-DNA-topoisomerase I. The ability of selected compounds to stimulate the topoisomerase I-mediated DNA cleavage and the persistence of the cleavable complex were consistent with the cytotoxic activity.

Regioselective synthesis and cytotoxicities of camptothecin derivatives modified at the 7-, 10- and 20-positions

A series of 7-acyloxymethylcamptothecin and 20-O-acyl-7-acyloxymethylcamptothecin derivatives were regioselectively prepared on different solvents. 7-Acyloxymethylcamptothecins possess more efficacy than 20-O-acyl-7-acyloxymethylcamptothecins against six human cancer cell lines in vitro.

Design and optimization of camptothecin conjugates of triple helix-forming oligonucleotides for sequence-specific DNA cleavage by topoisomerase I

To achieve a sequence-specific DNA cleavage by topoisomerase I, derivatives of the antitumor drug camptothecin have been covalently linked to triple helix-forming oligonucleotides that bind in a sequence-specific manner to the major groove of double-helical DNA. Triplex formation at the target sequence positions the drug selectively at the triplex site, thereby stimulating topoisomerase I-mediated DNA cleavage at this site. In a continuous effort to optimize this strategy, a broad set of conjugates consisting of (i) 16-20-base-long oligonucleotides, (ii) alkyl linkers of variable length, and (iii) camptothecin derivatives substituted on the A or B quinoline ring were designed and synthesized. Analysis of the cleavage sites at nucleotide resolution reveals that the specificity and efficacy of cleavage depends markedly on the length of both the triple-helical structure and the linker between the oligonucleotide and the poison. The optimized hybrid molecules induced strong and highly specific cleavage at a site adjacent to the triplex. Furthermore, the drug-stabilized DNA-topoisomerase I cleavage complexes were shown to be more resistant to salt-induced reversal than the complexes induced by camptothecin alone. Such rationally designed camptothecin conjugates could provide useful antitumor drugs directed selectively against genes bearing the targeted triplex binding site. In addition, they represent a powerful tool to probe the molecular interactions in the DNA-topoisomerase I complex.

Novel cytotoxic 7-iminomethyl and 7-aminomethyl derivatives of camptothecin

A series of new 7-iminomethyl derivatives of camptothecin were obtained from camptothecin-7-aldehyde and aromatic, alicyclic and aliphatic amines. Their hydrogenation led to the corresponding amines. All the imines and the less polar amines showed a marked increase of the cytotoxic activity against H460 non-small lung carcinoma cell line, with respect to topotecan. The lipophilicity of the substituent in position 7 of camptothecin seems to play an important role for cytotoxic potency. The 7-phenyliminomethyl derivative showed efficacy comparable to topotecan in vivo against NSCLC H460 xenografted in athymic nude mice.

Novel C-ring analogues of 20(S)-camptothecin. Part 3: synthesis and their in vitro cytotoxicity of A-, B- and C-ring analogues

Several 5-substituted alkoxy 20(S)-camptothecin analogues having A- and B-ring substituents were prepared via semi-synthesis. Most of these compounds were found to exhibit potent anti-cancer activity based on their in vitro cytotoxicity data obtained against human tumor cell lines.

In vitro cytotoxicity of 5-aminosubstituted 20(S)-camptothecins. Part 1

A number of 5-aminosubstituted 20(S)-camptothecin analogues were prepared via semi-synthesis starting from 20(S)-camptothecin and 9-methoxy 20(S)-camptothecin. In vitro anti-cancer activity of these analogues was determined using 60 human tumor cell line assay. Although water solubility of most of these compounds was improved compared to 20(S)-camptothecin, their anti-cancer activity was considerably diminished. However, only smaller substituents such as methylamine or hydroxylamine as present in 8s and 8t, respectively, showed good activity with improved water solubility.