Prodrugs of a 1-Hydroxy-2-oxopiperidin-3-yl Phosphonate Enolase Inhibitor for the Treatment of ENO1-Deleted Cancers.

Cancers harboring homozygous deletion of the glycolytic enzyme enolase 1 (ENO1) are selectively vulnerable to inhibition of the paralogous isoform, enolase 2 (ENO2). A previous work described the sustained tumor regression activities of a substrate-competitive phosphonate inhibitor of ENO2, 1-hydroxy-2-oxopiperidin-3-yl phosphonate (HEX) (5), and its bis-pivaloyoxymethyl prodrug, POMHEX (6), in an ENO1-deleted intracranial orthotopic xenograft model of glioblastoma [Nature Metabolism 2020, 2, 1423-1426]. Due to poor pharmacokinetics of bis-ester prodrugs, this study was undertaken to identify potential non-esterase prodrugs for further development. Whereas phosphonoamidate esters were efficiently bioactivated in ENO1-deleted glioma cells, McGuigan prodrugs were not. Other strategies, including cycloSal and lipid prodrugs of 5, exhibited low micromolar IC50 values in ENO1-deleted glioma cells and improved stability in human serum over 6. The activity of select prodrugs was also probed using the NCI-60 cell line screen, supporting its use to examine the relationship between prodrugs and cell line-dependent bioactivation.

MP-Pt(IV): A MAOB-Sensitive Mitochondrial-Specific Prodrug for Treating Glioblastoma.

We have previously reported the in vitro and in vivo efficacy of N,N-bis(2-chloroethyl)-2-(1-methyl-1,2,3,6-tetrahydropyridin-4-yl)propenamide (MP-MUS), a prodrug that targeted the mitochondria of glioblastoma (GBM). The mitochondrial enzyme, monoamine oxidase B (MAOB), is highly expressed in GBM and oxidizes an uncharged methyl-tetrahydropyridine (MP-) moiety into the mitochondrially targeted cationic form, methyl-pyridinium (P+-). Coupling this MAOB-sensitive group to a nitrogen mustard produced a prodrug that damaged GBM mitochondria and killed GBM cells. Unfortunately, the intrinsic reactivity of the nitrogen mustard group and low solubility of MP-MUS precluded clinical development. In our second-generation prodrug, MP-Pt(IV), we coupled the MP group to an unreactive cisplatin precursor. The enzymatic conversion of MP-Pt(IV) to P+-Pt(IV) was tested using recombinant human MAOA and rhMAOB. The generation of cisplatin from Pt(IV) by ascorbate was studied optically and using mass spectroscopy. Efficacy toward primary GBM cells and tumors was studied in vitro and in an intracranial patient-derived xenograft mice GBM model. Our studies demonstrate that MP-Pt(IV) is selectively activated by MAOB. MP-Pt(IV) is highly toxic toward GBM cells in vitro MP-Pt(IV) toxicity against GBM is potentiated by elevating mitochondrial ascorbate and can be arrested by MAOB inhibition. In in vitro studies, sublethal MP-Pt(IV) doses elevated mitochondrial MAOB levels in surviving GBM cells. MP-Pt(IV) is a potent chemotherapeutic in intracranial patient-derived xenograft mouse models of primary GBM and potentiates both temozolomide and temozolomide-chemoradiation therapies. MP-Pt(IV) was well tolerated and is highly effective against GBM in both in vitro and in vivo models.

A "Clickable" Probe for Active MGMT in Glioblastoma Demonstrates Two Discrete Populations of MGMT.

Various pathways can repair DNA alkylation by chemotherapeutic agents such as temozolomide (TMZ). The enzyme O6-methylguanine methyltransferase (MGMT) removes O6-methylated DNA adducts, leading to the failure of chemotherapy in resistant glioblastomas. Because of the anti-chemotherapeutic activities of MGMT previously described, estimating the levels of active MGMT in cancer cells can be a significant predictor of response to alkylating agents. Current methods to detect MGMT in cells are indirect, complicated, time-intensive, or utilize molecules that require complex and multistep chemistry synthesis. Our design simulates DNA repair by the transfer of a clickable propargyl group from O6-propargyl guanine to active MGMT and subsequent attachment of fluorescein-linked PEG linker via "click chemistry." Visualization of active MGMT levels reveals discrete active and inactive MGMT populations with biphasic kinetics for MGMT inactivation in response to TMZ-induced DNA damage.

The 3S Enantiomer Drives Enolase Inhibitory Activity in SF2312 and Its Analogues.

We recently reported that SF2312 ((1,5-dihydroxy-2-oxopyrrolidin-3-yl)phosphonic acid), a phosphonate antibiotic with a previously unknown mode of action, is a potent inhibitor of the glycolytic enzyme, Enolase. SF2312 can only be synthesized as a racemic-diastereomeric mixture. However, co-crystal structures with Enolase 2 (ENO2) have consistently shown that only the (3S,5S)-enantiomer binds to the active site. The acidity of the alpha proton at C-3, which deprotonates under mildly alkaline conditions, results in racemization; thus while the separation of four enantiomeric intermediates was achieved via chiral High Performance Liquid Chromatography (HPLC) of the fully protected intermediate, deprotection inevitably nullified enantiopurity. To prevent epimerization of the C-3, we designed and synthesized MethylSF2312, ((1,5-dihydroxy-3-methyl-2-oxopyrrolidin-3-yl)phosphonic acid), which contains a fully-substituted C-3 alpha carbon. As a racemic-diastereomeric mixture, MethylSF2312 is equipotent to SF2312 in enzymatic and cellular systems against Enolase. Chiral HPLC separation of a protected MethylSF2312 precursor resulted in the efficient separation of the four enantiomers. After deprotection and inevitable re-equilibration of the anomeric C-5, (3S)-MethylSF2312 was up to 2000-fold more potent than (3R)-MethylSF2312 in an isolated enzymatic assay. This observation strongly correlates with biological activity in both human cancer cells and bacteria for the 3S enantiomer of SF2312. Novel X-ray structures of human ENO2 with chiral and racemic MethylSF2312 show that only (3S,5S)-enantiomer occupies the active site. Enolase inhibition is thus a direct result of binding by the (3S,5S)-enantiomer of MethylSF2312. Concurrent with these results for MethylSF2312, we contend that the (3S,5S)-SF2312 is the single active enantiomer of inhibitor SF2312.

PAM-OBG: A monoamine oxidase B specific prodrug that inhibits MGMT and generates DNA interstrand crosslinks, potentiating temozolomide and chemoradiation therapy in intracranial glioblastoma.

Via extensive analyses of genetic databases, we have characterized the DNA-repair capacity of glioblastoma with respect to patient survival. In addition to elevation of O6-methylguanine DNA methyltransferase (MGMT), down-regulation of three DNA repair pathways; canonical mismatch repair (MMR), Non-Homologous End-Joining (NHEJ), and Homologous Recombination (HR) are correlated with poor patient outcome. We have designed and tested both in vitro and in vivo, a monoamine oxidase B (MAOB) specific prodrug, PAM-OBG, that is converted by glioma MAOB into the MGMT inhibitor O6-benzylguanine (O6BG) and the DNA crosslinking agent acrolein. In cultured glioma cells, we show that PAM-OBG is converted to O6BG, inhibiting MGMT and sensitizing cells to DNA alkylating agents such as BCNU, CCNU, and Temozolomide (TMZ). In addition, we demonstrate that the acrolein generated is highly toxic in glioma treated with an inhibitor of Nucleotide Excision Repair (NER). In mouse intracranial models of primary human glioma, we show that PAM-OBG increases survival of mice treated with either BCNU or CCNU by a factor of six and that in a chemoradiation model utilizing six rounds of TMZ/2Gy radiation, pre-treatment with PAM-OBG more than doubled survival time.

Development and validation of chemical features-based proton-coupled folate transporter/activity and reduced folate carrier/activity models (pharmacophores).

All clinically used antifolates lack transport selectivity for tumors over normal cells resulting in dose-limiting toxicities. There is growing interest in developing novel tumor-targeted cytotoxic antifolates with selective transport into tumors over normal cells via the proton-coupled folate transporter (PCFT) over the ubiquitously expressed reduced folate carrier (RFC). A lack of X-ray crystal structures or predictive models for PCFT or RFC has hindered structure-aided drug design for PCFT-selective therapeutics. Four-point validated models (pharmacophores) were generated for PCFT/Activity (HBA, NI, RA, RA) and RFC/Activity (HBD, NI, HBA, HBA) based on inhibition (IC50) of proliferation of isogenic Chinese hamster ovary (CHO) cells engineered to express only human PCFT or only RFC. Our results revealed substantial differences in structural features required for transport of novel molecules by these transporters which can be utilized for developing transporter-selective antifolates.

ENOblock Does Not Inhibit the Activity of the Glycolytic Enzyme Enolase.

Inhibition of glycolysis is of great potential for the treatment of cancer. However, inhibitors of glycolytic enzymes with favorable pharmacological profiles have not been forthcoming. Due to the nature of their active sites, most high-affinity transition-state analogue inhibitors of glycolysis enzymes are highly polar with poor cell permeability. A recent publication reported a novel, non-active site inhibitor of the glycolytic enzyme Enolase, termed ENOblock (N-[2-[2-2-aminoethoxy)ethoxy]ethyl]4-4-cyclohexylmethyl)amino]6-4-fluorophenyl)methyl]amino]1,3,5-triazin-2-yl]amino]benzeneacetamide). This would present a major advance, as this is heterocyclic and fully cell permeable molecule. Here, we present evidence that ENOblock does not inhibit Enolase enzymatic activity in vitro as measured by three different assays, including a novel 31P NMR based method which avoids complications associated with optical interferences in the UV range. Indeed, we note that due to strong UV absorbance, ENOblock interferes with the direct spectrophotometric detection of the product of Enolase, phosphoenolpyruvate. Unlike established Enolase inhibitors, ENOblock does not show selective toxicity to ENO1-deleted glioma cells in culture. While our data do not dispute the biological effects previously attributed to ENOblock, they indicate that such effects must be caused by mechanisms other than direct inhibition of Enolase enzymatic activity.

Tumor Targeting with Novel 6-Substituted Pyrrolo [2,3-d] Pyrimidine Antifolates with Heteroatom Bridge Substitutions via Cellular Uptake by Folate Receptor α and the Proton-Coupled Folate Transporter and Inhibition of de Novo Purine Nucleotide Biosynthesis.

Targeted antifolates with heteroatom replacements of the carbon vicinal to the phenyl ring in 1 by N (4), O (8), or S (9), or with N-substituted formyl (5), acetyl (6), or trifluoroacetyl (7) moieties, were synthesized and tested for selective cellular uptake by folate receptor (FR) α and ß or the proton-coupled folate transporter. Results show increased in vitro antiproliferative activity toward engineered Chinese hamster ovary cells expressing FRs by 4-9 over the CH2 analogue 1. Compounds 4-9 inhibited de novo purine biosynthesis and glycinamide ribonucleotide formyltransferase (GARFTase). X-ray crystal structures for 4 with FRα and GARFTase showed that the bound conformations of 4 required flexibility for attachment to both FRα and GARFTase. In mice bearing IGROV1 ovarian tumor xenografts, 4 was highly efficacious. Our results establish that heteroatom substitutions in the 3-atom bridge region of 6-substituted pyrrolo[2,3-d]pyrimidines related to 1 provide targeted antifolates that warrant further evaluation as anticancer agents.

6-Substituted Pyrrolo[2,3-d]pyrimidine Thienoyl Regioisomers as Targeted Antifolates for Folate Receptor α and the Proton-Coupled Folate Transporter in Human Tumors.

2-Amino-4-oxo-6-substituted-pyrrolo[2,3-d]pyrimidine antifolate thiophene regioisomers of AGF94 (4) with a thienoyl side chain and three-carbon bridge lengths [AGF150 (5) and AGF154 (7)] were synthesized as potential antitumor agents. These analogues inhibited proliferation of Chinese hamster ovary (CHO) sublines expressing folate receptors (FRs) α or ß (IC50s < 1 nM) or the proton-coupled folate transporter (PCFT) (IC50 < 7 nM). Compounds 5 and 7 inhibited KB, IGROV1, and SKOV3 human tumor cells at subnanomolar concentrations, reflecting both FRα and PCFT uptake. AGF152 (6) and AGF163 (8), 2,4-diamino-5-substituted-furo[2,3-d]pyrimidine thiophene regioisomers, also inhibited growth of FR-expressing CHO and KB cells. All four analogues inhibited glycinamide ribonucleotide formyltransferase (GARFTase). Crystal structures of human GARFTase complexed with 5 and 7 were reported. In severe combined immunodeficient mice bearing SKOV3 tumors, 7 was efficacious. The selectivity of these compounds for PCFT and for FRα and ß over the ubiquitously expressed reduced folate carrier is a paradigm for selective tumor targeting.

The design, synthesis and biological evaluation of conformationally restricted 4-substituted-2,6-dimethylfuro[2,3-d]pyrimidines as multi-targeted receptor tyrosine kinase and microtubule inhibitors as potential antitumor agents.

A series of eleven conformationally restricted, 4-substituted 2,6-dimethylfuro[2,3-d]pyrimidines was designed to explore the bioactive conformation required for dual inhibition of microtubule assembly and receptor tyrosine kinases (RTKs), and their biological activities are reported. All three rotatable single bonds in the lead compound 1 were sequentially restricted to address the role of each in SAR for microtubule and RTK inhibitory effects. Compounds 2, 3, 7 and 10 showed microtubule depolymerizing activity comparable to or better than the lead 1, some with nanomolar EC50 values. While compound 8 had no effect on microtubules, 8 and 10 both showed potent RTK inhibition with nanomolar IC50s. These compounds confirm that the bioactive conformation for RTK inhibition is different from that for tubulin inhibition. The tetrahydroquinoline analog 10 showed the most potent dual tubulin and RTK inhibitory activities (low nanomolar inhibition of EGFR, VEGFR2 and PDGFR-ß). Compound 10 has highly potent activity against many NCI cancer cell lines, including several chemo-resistant cell lines, and could serve as a lead for further preclinical studies.

Novel 5-substituted pyrrolo[2,3-d]pyrimidines as dual inhibitors of glycinamide ribonucleotide formyltransferase and 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase and as potential antitumor agents.

A new series of 5-substituted thiopheneyl pyrrolo[2,3-d]pyrimidines 6-11 with varying chain lengths (n = 1-6) were designed and synthesized as hybrids of the clinically used anticancer drug pemetrexed (PMX) and our 6-substituted thiopheneyl pyrrolo[2,3-d]pyrimidines 2c and 2d with folate receptor (FR) α and proton-coupled folate transporter (PCFT) uptake specificity over the reduced folate carrier (RFC) and inhibition of de novo purine nucleotide biosynthesis at glycinamide ribonucleotide formyltransferase (GARFTase). Compounds 6-11 inhibited KB human tumor cells in the order 9 = 10 > 8 > 7 > 6 = 11. Compounds 8-10 were variously transported by FRα, PCFT, and RFC and, unlike PMX, inhibited de novo purine nucleotide rather than thymidylate biosynthesis. The antiproliferative effects of 8 and 9 appeared to be due to their dual inhibitions of both GARFTase and 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase. Our studies identify a unique structure-activity relationship for transport and dual target inhibition.

Structure-activity profiles of novel 6-substituted pyrrolo[2,3-d]pyrimidine thienoyl antifolates with modified amino acids for cellular uptake by folate receptors α and ß and the proton-coupled folate transporter.

Structure-activity relationships for cellular uptake and inhibition of cell proliferation were studied for 2-amino-4-oxo-6-substituted pyrrolo[2,3-d]pyrimidine thienoyl antifolates in which the terminal l-glutamate of the parent structure (7) was replaced by natural or unnatural amino acids. Compounds 7 and 10-13 were selectively inhibitory toward folate receptor (FR) α-expressing Chinese hamster ovary (CHO) cells. Antiproliferative effects of compounds 7 and 9-13 toward FRα- and FRß-expressing CHO cells were only partly reflected in binding affinities to FRα and FRß or in the docking scores with molecular models of FRα and FRß. Compounds 7 and 11 were potent inhibitors of glycinamide ribonucleotide formyltransferase in de novo purine biosynthesis in KB human tumor cells. These studies establish for the first time the importance of the α- and γ-carboxylic acid groups, the length of the amino acid, and the conformation of the side chain for transporter binding and biological activity of 6-substituted pyrrolo[2,3-d]pyrimidine thienoyl antifolates.

The design and discovery of water soluble 4-substituted-2,6-dimethylfuro[2,3-d]pyrimidines as multitargeted receptor tyrosine kinase inhibitors and microtubule targeting antitumor agents.

The design, synthesis and biological evaluations of fourteen 4-substituted 2,6-dimethylfuro[2,3-d]pyrimidines are reported. Four compounds (11-13, 15) inhibit vascular endothelial growth factor receptor-2 (VEGFR-2), platelet-derived growth factor receptor ß (PDGFR-ß), and target tubulin leading to cytotoxicity. Compound 11 has nanomolar potency, comparable to sunitinib and semaxinib, against tumor cell lines overexpressing VEGFR-2 and PDGFR-ß. Further, 11 binds at the colchicine site on tubulin, depolymerizes cellular microtubules and inhibits purified tubulin assembly and overcomes both ßIII-tubulin and P-glycoprotein-mediated drug resistance, and initiates mitotic arrest leading to apoptosis. In vivo, its HCl salt, 21, reduced tumor size and vascularity in xenograft and allograft murine models and was superior to docetaxel and sunitinib, without overt toxicity. Thus 21 affords potential combination chemotherapy in a single agent.

Discovery of 5-substituted pyrrolo[2,3-d]pyrimidine antifolates as dual-acting inhibitors of glycinamide ribonucleotide formyltransferase and 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase in de novo purine nucleotide biosynthesis: implications of inhibiting 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase to ampk activation and antitumor activity.

We synthesized 5-substituted pyrrolo[2,3-d]pyrimidine antifolates (compounds 5-10) with one-to-six bridge carbons and a benozyl ring in the side chain as antitumor agents. Compound 8 with a 4-carbon bridge was the most active analogue and potently inhibited proliferation of folate receptor (FR) α-expressing Chinese hamster ovary and KB human tumor cells. Growth inhibition was reversed completely or in part by excess folic acid, indicating that FRα is involved in cellular uptake, and resulted in S-phase accumulation and apoptosis. Antiproliferative effects of compound 8 toward KB cells were protected by excess adenosine but not thymidine, establishing de novo purine nucleotide biosynthesis as the targeted pathway. However, 5-aminoimidazole-4-carboxamide (AICA) protection was incomplete, suggesting inhibition of both AICA ribonucleotide formyltransferase (AICARFTase) and glycinamide ribonucleotide formyltransferase (GARFTase). Inhibition of GARFTase and AICARFTase by compound 8 was confirmed by cellular metabolic assays and resulted in ATP pool depletion. To our knowledge, this is the first example of an antifolate that acts as a dual inhibitor of GARFTase and AICARFTase as its principal mechanism of action.

Tumor-targeting with novel non-benzoyl 6-substituted straight chain pyrrolo[2,3-d]pyrimidine antifolates via cellular uptake by folate receptor α and inhibition of de novo purine nucleotide biosynthesis.

A new series of 6-substituted straight side chain pyrrolo[2,3-d]pyrimidines 3a-d with varying chain lengths (n = 5-8) was designed and synthesized as part of our program to provide targeted antitumor agents with folate receptor (FR) cellular uptake specificity and glycinamide ribonucleotide formyltransferase (GARFTase) inhibition. Carboxylic acids 4a-d were converted to the acid chlorides and reacted with diazomethane, followed by 48% HBr to generate the α-bromomethylketones 5a-d. Condensation of 2,4-diamino-6-hydroxypyrimidine 6 with 5a-d afforded the 6-substituted pyrrolo[2,3-d]pyrimidines 7a-d. Hydrolysis and subsequent coupling with diethyl l-glutamate and saponification afforded target compounds 3a-d. Compounds 3b-d showed selective cellular uptake via FRα and -ß, associated with high affinity binding and inhibition of de novo purine nucleotide biosynthesis via GARFTase, resulting in potent inhibition against FR-expressing Chinese hamster cells and human KB tumor cells in culture. Our studies establish, for the first time, that a side chain benzoyl group is not essential for tumor-selective drug uptake by FRα.

Structure-activity relationship and in vitro and in vivo evaluation of the potent cytotoxic anti-microtubule agent N-(4-methoxyphenyl)-N,2,6-trimethyl-6,7-dihydro-5H-cyclopenta[d]pyrimidin-4-aminium chloride and its analogues as antitumor agents.

A series of 21 substituted cyclopenta[d]pyrimidines were synthesized as an extension of our discovery of the parent compound (±)-1·HCl as an anti-microtubule agent. The structure-activity relationship indicates that the N-methyl and a 4N-methoxy groups appear important for potent activity. In addition, the 6-substituent in the parent analogue is not necessary for activity. The most potent compound 30·HCl was a one to two digit nanomolar inhibitor of most tumor cell proliferations and was up to 7-fold more potent than the parent compound (±)-1·HCl. In addition, 30·HCl inhibited cancer cell proliferation regardless of Pgp or ßIII-tubulin status, both of which are known to cause clinical resistance to several anti-tubulin agents. In vivo efficacy of 30·HCl was demonstrated against a triple negative breast cancer xenograft mouse model. Compound 30·HCl is water-soluble and easily synthesized and serves as a lead compound for further preclinical evaluation as an antitumor agent.

Design, synthesis, and molecular modeling of novel pyrido[2,3-d]pyrimidine analogues as antifolates; application of Buchwald-Hartwig aminations of heterocycles.

Opportunistic infections caused by Pneumocystis jirovecii (P. jirovecii, pj), Toxoplasma gondii (T. gondii, tg), and Mycobacterium avium (M. avium, ma) are the principal causes of morbidity and mortality in patients with acquired immunodeficiency syndrome (AIDS). The absence of any animal models for human Pneumocystis jirovecii pneumonia and the lack of crystal structures of pjDHFR and tgDHFR make the design of inhibitors challenging. A novel series of pyrido[2,3-d]pyrimidines as selective and potent DHFR inhibitors against these opportunistic infections are presented. Buchwald-Hartwig coupling reaction of substituted anilines with pivaloyl protected 2,4-diamino-6-bromo-pyrido[2,3-d]pyrimidine was successfully explored to synthesize these analogues. Compound 26 was the most selective inhibitor with excellent potency against pjDHFR. Molecular modeling studies with a pjDHFR homology model explained the potency and selectivity of 26. Structural data are also reported for 26 with pcDHFR and 16 and 22 with variants of pcDHFR.

Synthesis and biological activity of 5-chloro-N4-substituted phenyl-9H-pyrimido[4,5-b]indole-2,4-diamines as vascular endothelial growth factor receptor-2 inhibitors and antiangiogenic agents.

Inhibition of receptor tyrosine kinase (RTK) signaling pathways is an important area for the development of novel anticancer agents. Numerous multikinase inhibitors (MKIs) have been recently approved for the treatment of cancer. Vascular endothelial growth factor receptor-2 (VEGFR-2) is the principal mediator of tumor angiogenesis. In an effort to develop ATP-competitive VEGFR-2 selective inhibitors the 5-chloro-N(4)-substituted phenyl-9H-pyrimido[4,5-b]indole-2,4-diamine scaffold was designed. The synthesis of the target compounds involved N-(4,5-dichloro-9H-pyrimido[4,5-b]indol-2-yl)-2,2-dimethylpropanamide) as a common intermediate. A nucleophilic displacement of the 4-chloro group of the common intermediate by appropriately substituted anilines afforded the target compounds. Biological evaluation indicated that compound 5 is a potent and selective VEGFR-2 inhibitor comparable to sunitinib and semaxinib.

Synthesis of N(4)-(substituted phenyl)-N(4)-alkyl/desalkyl-9H-pyrimido[4,5-b]indole-2,4-diamines and identification of new microtubule disrupting compounds that are effective against multidrug resistant cells.

A series of fourteen N(4)-(substituted phenyl)-N(4)-alkyl/desalkyl-9H-pyrimido[4,5-b]indole-2,4-diamines was synthesized as potential microtubule targeting agents. The synthesis involved a Fisher indole cyclization of 2-amino-6-hydrazinylpyrimidin-4(3H)-one with cyclohexanone, followed by oxidation, chlorination and displacement with appropriate anilines. Compounds 6, 14 and 15 had low nanomolar potency against MDA-MB-435 tumor cells and depolymerized microtubules. Compound 6 additionally had nanomolar GI(50) values against 57 of the NCI 60-tumor panel cell lines. Mechanistic studies showed that 6 inhibited tubulin polymerization and [(3)H]colchicine binding to tubulin. The most potent compounds were all effective in cells expressing P-glycoprotein or the ßIII isotype of tubulin, which have been associated with clinical drug resistance. Modeling studies provided the potential interactions of 6, 14 and 15 within the colchicine site.