Replacing the tropolonic methoxyl group of colchicine with methylamino increases tubulin binding affinity with improved therapeutic index and overcomes paclitaxel cross-resistance

AIMS

Microtubule inhibitors are widely used in first line cancer therapy, though drug resistance often develops and causes treatment failure. Colchicine binds to tubulins and inhibits tumor growth, but is not approved for cancer therapy due to systemic toxicity. In this study, we aim to improve the therapeutic index of colchicine through structural modification.

METHODS

The methoxyl group of the tropolonic ring in colchicine was replaced with amino groups. The cross-resistance of the derivatives with paclitaxel and vincristine was tested. Antitumor effects of target compounds were tested in vivo in A549 and paclitaxel-resistant A549/T xenografts. The interaction of target compounds with tubulins was measured using biological and chemical methods.

RESULTS

Methylamino replacement of the tropolonic methoxyl group of colchicine increases, while demethylation loses, selective tubulin binding affinity, G₂/M arrest and antiproliferation activity. Methylaminocolchicine is more potent than paclitaxel and vincristine to inhibit tumor growth in vitro and in vivo without showing cross-resistance to paclitaxel. Methylaminocolchicine binds to tubulins in unique patterns and inhibits P-gp with a stable pharmacokinetic profile.

CONCLUSION

Methylanimo replacement of the tropolonic methoxyl group of colchicine increases antitumor activity with improved therapeutic index. Methylaminocolchicine represents a new type of mitotic inhibitor with the ability of overcoming paclitaxel and vincristine resistance.

Rational design, chemical synthesis and cellular evaluation of novel 1,3-diynyl derivatives of noscapine as potent tubulin binding anticancer agents

We present a new class of derivatives of noscapine, 1,3-diynyl-noscapinoids of an antitussive plant alkaloid, noscapine based on our in silico efforts that binds tubulin and displays anticancer activity against a panel of breast cancer cells. Structure-activity analyses pointed the C-9 position of the isoquinoline ring which was modified by coupling of 1,3-diynyl structural motifs to rationally design and screened a series of novel 1,3-diynyl-noscapinoids (20-22) with robust binding affinity with tubulin. The selected 1,3-diynyl-noscapinoids, 20-22 revealed improved predicted binding energy of -6.568 kcal/mol for 20, -7.367 kcal/mol for 21 and -7.922 kcal/mol for 22, respectively in comparison to the lead molecule (-5.246 kcal/mol). These novel derivatives were chemically synthesized and validated their anticancer activity based on cellular studies using two human breast adenocarcinoma, MCF-7 and MDAMB-231, as well as with a panel of primary breast cancer cells isolated from patients. Interestingly, all these derivatives inhibited cellular proliferation in all the cancer cells that ranged between 6.2 to 38.9 μM, which is 6.7 to 1.5 fold lower than that of noscapine. Unlike previously reported derivatives of noscapine that arrests cells in the S-phase, these novel derivatives effectively inhibit proliferation of cancer cells, arrests cell cycle in the G2/M-phase followed by apoptosis and appearance of apoptotic cells. Thus, we conclude that 1,3-diynyl-noscapinoids have great potential to be a novel therapeutic agent for breast cancers.

Together, the IFT81 and IFT74 N-termini form the main module for intraflagellar transport of tubulin

The assembly and maintenance of most cilia and flagella rely on intraflagellar transport (IFT). Recent in vitro studies have suggested that, together, the calponin-homology domain within the IFT81 N-terminus and the highly basic N-terminus of IFT74 form a module for IFT of tubulin. By using Chlamydomonas mutants for IFT81 and IFT74, we tested this hypothesis in vivo. Modification of the predicted tubulin-binding residues in IFT81 did not significantly affect basic anterograde IFT and length of steady-state flagella but slowed down flagellar regeneration, a phenotype similar to that seen in a strain that lacks the IFT74 N-terminus. In both mutants, the frequency of tubulin transport by IFT was greatly reduced. A double mutant that combined the modifications to IFT81 and IFT74 was able to form only very short flagella. These results indicate that, together, the IFT81 and IFT74 N-termini are crucial for flagellar assembly, and are likely to function as the main module for IFT of tubulin.

Novel hybrid nocodazole analogues as tubulin polymerization inhibitors and their antiproliferative activity

We describe the design, synthesis and SAR profiling of a series of novel combretastatin-nocodazole conjugates as potential anticancer agents. The thiophene ring in the nocodazole moiety was replaced by a substituted phenyl ring from the combretastatin moiety to design novel hybrid analogues. The hydroxyl group at the ortho position in compounds 2, 3 and 4 was used as the conformationally locking tool by anticipated six-membered hydrogen bonding. The bioactivity profiles of all compounds as tubulin polymerization inhibitors and as antiproliferative agents against the A-549 human lung cancer cell line were investigated Compounds 1 and 4 showed μM IC50 values in both assays.

Synthesis, anticancer activity and molecular docking studies on a series of heterocyclic trans-cyanocombretastatin analogues as antitubulin agents

A series of heterocyclic combretastatin analogues have been synthesized and evaluated for their anticancer activity against a panel of 60 human cancer cell lines. The most potent compounds were two 3,4,5-trimethoxy phenyl analogues containing either an (Z)-indol-2-yl (8) or (Z)-benzo[b]furan-2-yl (12) moiety; these compounds exhibited GI50 values of <10 nM against 74% and 70%, respectively, of the human cancer cell lines in the 60-cell panel. Compounds 8, and 12 and two previously reported compounds in the same structural class, i.e. 29 and 31, also showed potent anti-leukemic activity against leukemia MV4-11 cell lines with LD50 values = 44 nM, 47 nM, 18 nM, and 180 nM, respectively. From the NCI anti-cancer screening results and the data from the in vitro toxicity screening on cultured AML cells, seven compounds: 8, 12, 21, 23, 25, 29 and 31 were screened for their in vitro inhibitory activity on tubulin polymerization in MV4-11 AML cells; at 50 nM, 8 and 29 inhibited polymerization of tubulin by >50%. The binding modes of the three most active compounds (8, 12 and 29) to tubulin were also investigated utilizing molecular docking studies. All three molecules were observed to bind in the same hydrophobic pocket at the interface of α- and β-tubulin that is occupied by colchicine, and were stabilized by van der Waals' interactions with surrounding tubulin residues. The results from the tubulin polymerization and molecular docking studies indicate that compounds 8 and 29 are the most potent anti-leukemic compounds in this structural class, and are considered lead compounds for further development as anti-leukemic drugs.

Novel third-generation water-soluble noscapine analogs as superior microtubule-interfering agents with enhanced antiproliferative activity

Noscapine, an opium-derived 'kinder-gentler' microtubule-modulating drug is in Phase I/II clinical trials for cancer chemotherapy. However, its limited water solubility encumbers its development into an oral anticancer drug with clinical promise. Here we report the synthesis of 9 third-generation, water-soluble noscapine analogs with negatively charged sulfonato and positively charged quaternary ammonium groups using noscapine, 9-bromonoscapine and 9-aminonoscapine as scaffolds. The predictive free energy of solvation was found to be lower for sulfonates (6a-c; 8a-c) compared to the quaternary ammonium-substituted counterparts, explaining their higher water solubility. In addition, sulfonates showed higher charge dispersability, which may effectively shield the hydrophobicity of isoquinoline nucleus as indicated by hydrophobicity mapping methods. These in silico data underscore efficient net charge balancing, which may explain higher water solubility and thus enhanced antiproliferative efficacy and improved bioavailability. We observed that 6b, 8b and 8c strongly inhibited tubulin polymerization and demonstrated significant antiproliferative activity against four cancer cell lines compared to noscapine. Molecular simulation and docking studies of tubulin-drug complexes revealed that the brominated compound with a four-carbon chain (4b, 6b, and 8b) showed optimal binding with tubulin heterodimers. Interestingly, 6b, 8b and 8c treated PC-3 cells resulted in preponderance of mitotic cells with multipolar spindle morphology, suggesting that they stall the cell cycle. Furthermore, in vivo pharmacokinetic evaluation of 6b, 8b and 8c revealed at least 1-2-fold improvement in their bioavailability compared to noscapine. To our knowledge, this is the first report to demonstrate novel water-soluble noscapine analogs that may pave the way for future pre-clinical drug development.