Frank Loganzo

Paclitaxel-resistant cells have a mutation in the paclitaxel-binding region of β-tubulin (Asp26Glu) and less stable microtubules

Resistance to paclitaxel-based therapy is frequently encountered in the clinic. The mechanisms of intrinsic or acquired paclitaxel resistance are not well understood. We sought to characterize the resistance mechanisms that develop upon chronic exposure of a cancer cell line to paclitaxel in the presence of the P-glycoprotein reversal agent, CL-347099. The epidermoid tumor line KB-3-1 was exposed to increasing concentrations of paclitaxel and 5 μmol/L CL-347099 for up to 1 year. Cells grown in 15 nmol/L paclitaxel plus CL-347099 (KB-15-PTX/099) developed 18-fold resistance to paclitaxel and were dependent upon paclitaxel for maximal growth. They grew well and retained resistance to paclitaxel when grown in athymic mice. Cross-resistance (3- to 5-fold) was observed in tissue culture to docetaxel, the novel taxane MAC-321, and epothilone B. Collateral sensitivity (∼3-fold) was observed to the depolymerizing agents vinblastine, dolastatin-10, and HTI-286. KB-15-PTX/099–resistant cells did not overexpress P-glycoprotein nor did they have an alteration of [14C]paclitaxel accumulation compared with parental cells. However, a novel point mutation (T to A) resulting in Asp26 to glutamate substitution in class I (M40) β-tubulin was found. Based on an electron crystallography structure of Zn-stabilized tubulin sheets, the phenyl ring of C-3′ NHCO-C6H5 of paclitaxel makes contact with Asp26 of β-tubulin, suggesting a ligand-induced mutation. Optimized model complexes of paclitaxel, docetaxel, and MAC-321 in β-tubulin show a novel hydrogen bonding pattern for the glutamate mutant and rationalize the observed resistance profiles. However, a mutation in the paclitaxel binding pocket does not explain the phenotype completely. KB-15-PTX/099 cells have impaired microtubule stability as determined by a reduced percentage of tubulin in microtubules and reflected by less acetylated tubulin. These results suggest that a mutation in tubulin might affect microtubule stability as well as drug binding and contribute to the observed resistance profile. [Mol Cancer Ther 2006;5(2):270–8]

Targeting vascular and avascular compartments of tumors with C. novyi-NT and anti-microtubule agents.

Current approaches for treating cancer are limited, in part, by the inability of drugs to affect the poorly vascularized regions of tumors. We have found that C. novyi-NT in combination with anti-microtubule agents can cause the destruction of both the vascular and avascular compartments of tumors. The two classes of microtubule inhibitors were found to exert markedly different effects. Some agents that inhibited microtubule synthesis, such as HTI-286 and vinorelbine, caused rapid, massive hemorrhagic necrosis when used in combination with C. novyi-NT. In contrast, agents that stabilized microtubules, such as the taxanes docetaxel and MAC-321, resulted in slow tumor regressions that killed most neoplastic cells. Remaining cells in the poorly perfused regions of tumors could be eradicated by C. novyi-NT. Mechanistic studies showed that the microtubule destabilizers, but not the microtubule stabilizers, radically reduced blood flow to tumors, thereby enlarging the hypoxic niche in which C. novyi-NT spores could germinate. A single intravenous injection of C. novyi-NT plus selected anti-microtubule agents was able to cause regressions of several human tumor xenografts in nude mice in the absence of excessive toxicity.

Discovery of Cytotoxic Dolastatin 10 Analogues with N-Terminal Modifications

Auristatins, synthetic analogues of the antineoplastic natural product Dolastatin 10, are ultrapotent cytotoxic microtubule inhibitors that are clinically used as payloads in antibody-drug conjugates (ADCs). The design and synthesis of several new auristatin analogues with N-terminal modifications that include amino acids with α,α-disubstituted carbon atoms are described, including the discovery of our lead auristatin, PF-06380101. This modification of the peptide structure is unprecedented and led to analogues with excellent potencies in tumor cell proliferation assays and differential ADME properties when compared to other synthetic auristatin analogues that are used in the preparation of ADCs. In addition, auristatin cocrystal structures with tubulin are being presented that allow for the detailed examination of their binding modes. A surprising finding is that all analyzed analogues have a cis-configuration at the Val-Dil amide bond in their functionally relevant tubulin bound state, whereas in solution this bond is exclusively in the trans-configuration. This remarkable observation shines light onto the preferred binding mode of auristatins and serves as a valuable tool for structure-based drug design.

Total Syntheses and Biological Reassessment of Lactimidomycin, Isomigrastatin and Congener Glutarimide Antibiotics

Lactimidomycin (1) was described in the literature as an exquisitely potent cell migration inhibitor. Encouraged by this claim, we developed a concise and scalable synthesis of this bipartite glutarimide-macrolide antibiotic, which relies on the power of ring-closing alkyne metathesis (RCAM) for the formation of the unusually strained 12-membered head group. Subsequent deliberate digression from the successful path to 1 also brought the sister compound isomigrastatin (2) as well as a series of non-natural analogues of these macrolides into reach. A careful biological re-evaluation of this compound collection showed 1 and progeny to be potently cytotoxic against a panel of cancer cell lines, even after one day of compound exposure; therefore any potentially specific effects on tumor cell migration were indistinguishable from the acute effect of cell death. No significant cell migration inhibition was observed at sub-toxic doses. Although these findings cannot be reconciled with some reports in the literature, they are in accord with the notion that lactimidomycin is primarily a ribosome-binder able to effectively halt protein biosynthesis at the translation stage.

Tumor Cells Chronically Treated with a Trastuzumab–Maytansinoid Antibody–Drug Conjugate Develop Varied Resistance Mechanisms but Respond to Alternate Treatments

Antibody–drug conjugates (ADC) are emerging as clinically effective therapy. We hypothesized that cancers treated with ADCs would acquire resistance mechanisms unique to immunoconjugate therapy and that changing ADC components may overcome resistance. Breast cancer cell lines were exposed to multiple cycles of anti-Her2 trastuzumab–maytansinoid ADC (TM-ADC) at IC80 concentrations followed by recovery. The resistant cells, 361-TM and JIMT1-TM, were characterized by cytotoxicity, proteomic, transcriptional, and other profiling. Approximately 250-fold resistance to TM-ADC developed in 361-TM cells, and cross-resistance was observed to other non–cleavable-linked ADCs. Strikingly, these 361-TM cells retained sensitivity to ADCs containing cleavable mcValCitPABC-linked auristatins. In JIMT1-TM cells, 16-fold resistance to TM-ADC developed, with cross-resistance to other trastuzumab-ADCs. Both 361-TM and JIMT1-TM cells showed minimal resistance to unconjugated mertansine (DM1) and other chemotherapeutics. Proteomics and immunoblots detected increased ABCC1 (MRP1) drug efflux protein in 361-TM cells, and decreased Her2 (ErbB2) in JIMT1-TM cells. Proteomics also showed alterations in various pathways upon chronic exposure to the drug in both cell models. Tumors derived from 361-TM cells grew in mice and were refractory to TM-ADC compared with parental cells. Hence, acquired resistance to trastuzumab–maytansinoid ADC was generated in cultured cancer cells by chronic drug treatment, and either increased ABCC1 protein or reduced Her2 antigen were primary mediators of resistance. These ADC-resistant cell models retain sensitivity to other ADCs or standard-of-care chemotherapeutics, suggesting that alternate therapies may overcome acquired ADC resistance. Mol Cancer Ther; 14(4); 952–63. ©2015 AACR.

Synthesis, molecular editing, and biological assessment of the potent cytotoxin leiodermatolide.

It was by way of total synthesis that the issues concerning the stereostructure of leiodermatolide (1) have recently been solved; with the target now being unambiguously defined, the mission of synthesis changes as to secure a meaningful supply of this exceedingly scarce natural product derived from a deep-sea sponge. To this end, a scalable route of 19 steps (longest linear sequence) has been developed, which features a catalytic asymmetric propargylation of a highly enolizable β-keto-lactone, a ring closing alkyne metathesis and a modified Stille coupling as the key transformations. Deliberate digression from this robust blueprint brought a first set of analogues into reach, which allowed the lead qualities of 1 to be assessed. The acquired biodata show that 1 is a potent cytotoxin in human tumor cell proliferation assays, distinguished by GI50 values in the ≤3 nM range even for cell lines expressing the Pgp efflux transporter. Studies with human U2OS cells revealed that 1 causes mitotic arrest, micronucleus induction, centrosome amplification and tubulin disruption, even though no evidence for direct tubulin binding has been found in cell-free assays; moreover, the compound does not seem to act through kinase inhibition. Indirect evidence points at centrosome declustering as a possible mechanism of action, which provides a potentially rewarding outlook in that centrosome declustering agents hold promise of being inherently selective for malignant over healthy human tissue.

Combining antibody–drug conjugates and immune-mediated cancer therapy: What to expect?

Blockade of immune-checkpoints has emerged as one of the most promising approaches to improve the durability of anti-tumor responses in cancer patients. However, the fraction of patients experiencing durable responses to single agent immune checkpoint inhibitor treatment remains limited. Recent clinical reports suggest that patients responding best to checkpoint blockade therapies display higher levels of CD8(+) T-cells in the tumor prior to treatment. Therefore, combination treatments of immune-checkpoint inhibitors with compounds that increase the number of tumor infiltrating CD8(+) T cells may expand the therapeutic benefit of immuno-oncology (IO) drugs. Immunogenic cell death (ICD) of tumor cells is induced by certain classes of cytotoxic compounds and represents a potent stimulator of effector T-cell recruitment to tumors. In addition, several cytotoxics directly stimulate dendritic cell activation and maturation, resulting in improved anti-tumor immune responses when combined with IO compounds. Among them, several cytotoxic agents are currently utilized as payloads for antibody-drug conjugates (ADCs). Therefore, identification of optimal combination regimens between ADC- and IO compounds holds strong promise to overcome the current limitations of immune checkpoint inhibitors, by increasing the recruitment of CD8(+) effector T-cells to the tumor core. Here we review the emerging field of ADC/IO combination research, with a focus on how to optimally combine both modalities. The answer to this question may have a broader impact on oncology drug development, as synergistic activities between IO compounds and ADCs may increase the formation of tumor specific immunological memory, ultimately leading to durable responses in a larger fraction of cancer patients.

Mechanisms of Resistance to Antibody-Drug Conjugates.

Drug resistance limits the effectiveness of cancer therapies. Despite attempts to develop curative anticancer treatments, tumors evolve evasive mechanisms limiting durable responses. Hence, diverse therapies are used to attack cancer, including cytotoxic and targeted agents. Antibody–drug conjugates (ADC) are biotherapeutics designed to deliver potent cytotoxins to cancer cells via tumor-specific antigens. Little is known about the clinical manifestations of drug resistance to this class of therapy; however, recent preclinical studies reveal potential mechanisms of resistance. Because ADCs are a combination of antibody and small molecule cytotoxin, multifactorial modes of resistance are emerging that are inherent to the structure and function of the ADC. Decreased cell-surface antigen reduces antibody binding, whereas elevated drug transporters such as MDR1 and MRP1 reduce effectiveness of the payload. Inherent to the uniqueness of the ADC, other novel resistance mechanisms are emerging, including altered antibody trafficking, ADC processing, and intracellular drug release. Most importantly, the modular nature of the ADC allows components to be switched and replaced, enabling development of second-generation ADCs that overcome acquired resistance. This review is intended to highlight recent progress in our understanding of ADC resistance, including approaches to create preclinical ADC-refractory models and to characterize their emerging mechanisms of resistance. Mol Cancer Ther; 15(12); 2825–34. ©2016 AACR.

Cytotoxic Spliceostatins from Burkholderia sp. and Their Semisynthetic Analogues

The spliceostatin class of natural products was reported to be potent cytotoxic agents via inhibition of the spliceosome, a key protein complex in the biosynthesis of mature mRNA. As part of an effort to discover novel leads for cancer chemotherapy, we re-examined this class of compounds from several angles, including fermentation of the producing strains, isolation and structure determination of new analogues, and semisynthetic modification. Accordingly, a group of spliceostatins were isolated from a culture broth of Burkholderia sp. FERM BP-3421, and their structures identified by analysis of spectroscopic data. Semisynthesis was performed on the major components 4 and 5 to generate ester and amide derivatives with improved in vitro potency. With their potent activity against tumor cells and unique mode of action, spliceostatins can be considered potential leads for development of cancer drugs.

Development of Solid-Phase Site-Specific Conjugation and Its Application toward Generation of Dual Labeled Antibody and Fab Drug Conjugates.

The focus of the antibody-drug conjugate (ADC) field is shifting toward development of site-specific, next-generation ADCs to address the issue of heterogeneity, metabolic instability, conjugatability, and less than ideal therapeutic index associated with the conventional (heterogeneous) ADCs. It is evident from the recent literature that the site of conjugation, the structure of the linker, and the physicochemical properties of the linker-payload all have a significant impact on the safety and efficacy of the resulting ADCs. Screening multiple linker-payloads on multiple sites of an antibody presents a combinatorial problem that necessitates high-throughput conjugation and purification methodology to identify ADCs with the best combination of site and payload. Toward this end, we developed a protein A/L-based solid-phase, site-specific conjugation and purification method that can be used to generate site-specific ADCs in a 96-well plate format. This solid-phase method has been shown to be versatile because of its compatibility with various conjugation functional handles such as maleimides, haloacetamides, copper free click substrates, and transglutaminase substrates. The application of this methodology was further expanded to generate dual labeled, site-specific antibody and Fab conjugates.