Prostate-Specific Membrane Antigen-Specific Antitumor Activity of a Self-Immolative Tubulysin Conjugate.

Prostate-specific membrane antigen (PSMA) is a biomarker that is overexpressed on prostate cancer, and it is also present on the neovasculature within many non-prostate solid tumors. Herein, we report on the construction and biological testing of novel tubulysin B-containing therapeutic agents for the treatment of PSMA-expressing cancer. One of these compounds, EC1169, emerged as a lead candidate for preclinical development and phase 1 clinical testing. This water-soluble conjugate was shown to have high affinity for PSMA-positive cells. When tested in vitro, EC1169 was found to inhibit the growth of PSMA-positive cells, but it displayed no activity against PSMA-negative cells. Brief treatment of nude mice bearing PSMA-positive LNCaP human xenografts with EC1169 led to complete remissions and cures. Furthermore, this activity occurred in the absence of weight loss. In contrast, the nontargeted tubulysin B drug proved to be inactive against the LNCaP tumor model when administered at doses near to or greater than the maximum tolerated level. PSMA-negative KB tumors did not appreciably respond to EC1169 therapy, thereby confirming this compound's targeted specificity for PSMA-positive cells. Finally, treatment of LNCaP-bearing mice with docetaxel (the most active chemotherapeutic agent approved for late stage prostate cancer therapy) was found to produce only modest anti-tumor activity, and this outcome was also associated with severe weight loss. Taken together, these results strongly indicate that PSMA-positive tumors may be effectively treated using highly potent, PSMA-targeted small-molecule drug conjugates using regimens that do not cause undesirable side effects.

Pre-clinical evaluation of EC1456, a folate-tubulysin anti-cancer therapeutic.

EC1456 is a folate-tubulysin conjugate constructed with an all-D enantiomeric spacer/linker configuration. When tested against folate receptor (FR)-positive cells, EC1456 demonstrated dose-responsive activity with an approximate 1000-fold level of specificity. Treatment of nude mice bearing FR-positive human xenografts (as large as 800 mm3) with non-toxic doses of EC1456 led to cures in 100% of the mice. Combinations of low dose EC1456 with standard of care agents such as platins, taxanes, topotecan and bevacizumab, safely and significantly augmented the growth inhibitory effects of these commonly used agents. When tested against FR-positive human tumor xenograft models having confirmed resistance to a folate-vinca alkaloid (vintafolide), cisplatin or paclitaxel, EC1456 was found to generate partial to curative responses. Taken together, these studies demonstrate that EC1456 has significant anti-proliferative activity against FR-positive tumors, including models which were anticancer drug resistant, thereby justifying a Phase 1 trial of this agent for the treatment of advanced human cancers.

Enhancing the therapeutic range of a targeted small-molecule tubulysin conjugate for folate receptor-based cancer therapy.

PURPOSE: EC0305 represents a folate-tubulysin B construct capable of specifically eradicating folate receptor (FR)-positive subcutaneous tumors from mice (Leamon et al., Cancer Res 68:9839-9844, 8). Herein we report on the use of multiple polar carbohydrate segments (e.g. 1-amino-1-deoxy-glucitolyl-γ-glutamate) placed in-between the folate and tubulysin B moieties of EC0305 creating a new conjugate, herein referred to as EC0531, with more desirable biological properties. METHODS: The synthesis of EC0531 and its tritium-labeled counterpart are described. EC0531's affinity for FR binding and specific cytotoxic activity was assessed using standard in vitro assays. Human tumor xenografts were used to directly compare EC0305 and EC0531's antitumor activity. Finally, bile duct cannulated, female Sprague-Dawley rats were used to compare hepatobiliary clearance of these two targeted chemotherapeutic agents. RESULTS: EC0531 tightly binds to the FR with an affinity about half that of folic acid. It was found to specifically inhibit the growth of FR+ cells (IC50 of ~2 nM) in a dose-dependent manner. Using 3H-labeled compounds, more than a 12-fold higher amount of tubulysin was measured in a FR + human tumor xenograft compared to the unconjugated drug, a finding that explains, in part, why EC0531 displays curative activity, whereas the unconjugated tubulysin agent is essentially inactive. EC0531 was found to produce greater FR-specific anti-tumor activity at lower dose levels than EC0305; furthermore, EC0531's maximum tolerated dose level was significantly higher than that of EC0305, likely because EC0531's saccharopeptidic-based spacer allows for ~sixfold reduction in hepatic clearance. CONCLUSIONS: These data provide additional evidence that the therapeutic range of targeted small-molecule drug conjugates can be favorably increased using molecular spacers constructed with 1-amino-1-deoxy-glucitolyl-γ-glutamate residues.

High Levels of Expression of P-glycoprotein/Multidrug Resistance Protein Result in Resistance to Vintafolide.

Targeting surface receptors overexpressed on cancer cells is one way to specifically treat cancer versus normal cells. Vintafolide (EC145), which consists of folate linked to a cytotoxic small molecule, desacetylvinblastine hydrazide (DAVLBH), takes advantage of the overexpression of folate receptor (FR) on cancer cells. Once bound to FR, vintafolide enters the cell by endocytosis, and the reducing environment of the endosome cleaves the linker, releasing DAVLBH to destabilize microtubules. Vintafolide has shown efficacy and improved tolerability compared with DAVLBH in FR-positive preclinical models. As the first FR-targeting drug to reach the clinic, vintafolide has achieved favorable responses in phase II clinical trials in FR-positive ovarian and lung cancer. However, some FR-positive patients in these clinical trials do not respond to vintafolide. We sought to identify potential biomarkers of resistance to aid in the future development of this and other FR-targeting drugs. Here, we confirm that high P-glycoprotein (P-gp) expression was the strongest predictor of resistance to DAVLBH in a panel of 359 cancer cell lines. Furthermore, targeted delivery of DAVLBH via the FR, as in vintafolide, fails to overcome P-gp-mediated efflux of DAVLBH in both in vitro and in vivo preclinical models. Therefore, we suggest that patients whose tumors express high levels of P-gp be excluded from future clinical trials for vintafolide as well as other FR-targeted therapeutics bearing a P-gp substrate. Mol Cancer Ther; 15(8); 1998-2008. ©2016 AACR.

Folate-vinca alkaloid conjugates for cancer therapy: a structure-activity relationship.

Vintafolide is a potent folate-targeted vinca alkaloid small molecule drug conjugate (SMDC) that has shown promising results in multiple clinical oncology studies. Structurally, vintafolide consists of 4 essential modules: (1) folic acid, (2) a hydrophilic peptide spacer, (3) a disulfide-containing, self-immolative linker, and (4) the cytotoxic drug, desacetylvinblastine hydrazide (DAVLBH). Here, we report a structure-activity study evaluating the biological impact of (i) substituting DAVLBH within the vintafolide molecule with other vinca alkaloid analogues such as vincristine, vindesine, vinflunine, or vinorelbine; (ii) substituting the naturally (S)-configured Asp-Arg-Asp-Asp-Cys peptide with alternative hydrophilic spacers of varied composition; and (iii) varying the composition of the linker module. A series of vinca alkaloid-containing SMDCs were synthesized and purified by HPLC and LCMS. The SMDCs were screened in vitro against folate receptor (FR)-positive cells, and anti-tumor activity was tested against well-established subcutaneous FR-positive tumor xenografts. The cytotoxic and anti-tumor activity was directly compared to that produced by vintafolide. Among all the folate vinca alkaloid SMDCs tested, DAVLBH-containing SMDCs were active, while those constructed with vincristine, vindesine, or vinorelbine analogues failed to produce meaningful biological activity. Within the DAVLBH series, having a bioreleasable, self-immolative linker system was found to be critical for activity since multiple analogues constructed with thioether-based linkers all failed to produce meaningful activity both in vitro and in vivo. Substitutions of some or all of the natural amino acids within vintafolide's hydrophilic spacer module did not significantly change the in vitro or in vivo potency of the SMDCs. Vintafolide remains one of the most potent folate-vinca alkaloid SMDCs produced to date, and continued clinical development is warranted.

Rational combination therapy of vintafolide (EC145) with commonly used chemotherapeutic drugs.

PURPOSE: When evaluated in patients with ovarian and other cancer, vintafolide (EC145), a potent folate-targeted vinca alkaloid conjugate, displayed a toxicity profile that seemed to be nonoverlapping with many standard-of-care cancer therapeutics. It was, therefore, hypothesized that combining vintafolide with certain approved anticancer drugs may afford greater therapeutic efficacy compared with single-agent therapy. To explore this concept, vintafolide was evaluated in combination with pegylated liposomal doxorubicin (PLD; DOXIL), cisplatin, carboplatin, paclitaxel, docetaxel, topotecan, and irinotecan against folate receptor (FR)-positive models. EXPERIMENTAL DESIGN: FR-expressing KB, M109, IGROV, and L1210 cells were first exposed to graded concentrations of vintafolide, either alone or in combination with doxorubicin (active ingredient in PLD), and isobologram plots and combination index values generated. The vintafolide combinations were also studied in mice bearing various FR-expressing tumors. RESULTS: Vintafolide displayed strong synergistic activity against KB cells when combined with doxorubicin, and no less-than-additive effects resulted when tested against M109, IGROV, and L1210 cells. In contrast, when either desacetylvinblastine hydrazide (DAVLBH; the vinca alkaloid moiety in vintafolide) or vindesine (the vinca alkaloid most structurally similar to DAVLBH) were tested in combination with doxorubicin, less-than-additive antitumor effects were observed. In vivo, all vintafolide drug combinations produced far greater antitumor effect (complete responses and cures) compared with the single agents alone, without significant increase in overall toxicity. Importantly, these benefits were not observed with combinations of PLD and DAVLBH or vindesine. CONCLUSIONS: On the basis of these encouraging preclinical results, clinical studies to evaluate vintafolide drug combination therapies are now under way.

Reducing undesirable hepatic clearance of a tumor-targeted vinca alkaloid via novel saccharopeptidic modifications.

During a phase I trial of EC145 (a folate-targeted vinca alkaloid conjugate), constipation was identified as the dose-limiting toxicity, probably from a nonfolate receptor-related liver clearance process capable of releasing unconjugated vinca alkaloid from EC145 and shuttling it to the bile. Here, we report on the selective placement of novel carbohydrate segments (1-amino-1-deoxy-glucitolyl-γ-glutamate) spaced in-between the folate and vinca alkaloid moieties of EC145, which yielded a new agent (EC0489) that is equipotent but less toxic than EC145. Whereas both compounds could cure tumor-bearing mice reproducibly, EC0489 differed from EC145 with i) a shorter elimination half-life, ii) approximately 70% decrease in bile clearance, iii) a 4-fold increase in urinary excretion, and iv) improved tolerability in rodents. This combination of improvements justified the clinical evaluation of EC0489 where currently administered dose levels have exceeded the maximal tolerated dose of EC145 by approximately 70%, thereby reflecting the translational benefits to this new approach.

In vivo structural activity and optimization studies of folate-tubulysin conjugates.

Herein we report on the potencies of 4 related folate-conjugated tubulysins constructed with either tubulysin B hydrazide (EC0305), tubulysin A hydrazide (EC0510), the N,O-acetal derivative of natural tubulysins (EC0317) or a tubulysin B ester (EC0302). Our results confirmed that EC0305 is the most favorable conjugate of the group due to its potent antitumor activity [100% cures at 1 micromol/kg, three times a week (TIW) for 2 weeks] and its favorably low toxicity profile. In contrast, the natural tubulysin B drug proved to be inactive against a human nasopharyngeal tumor model when administered at doses near to or greater than the maximum tolerated dose (MTD). When tested against more chemoresistant folate receptor expressing M109 and 4T1-cl2 tumors, EC0305 displayed superior antitumor activity over a previously disclosed folate conjugate of desacetylvinblastine monohydrazide (EC145). These studies demonstrate that EC0305 has significant antiproliferative activity against FR expressing tumors, including those which are generally more chemoresistant, and that EC0305 should be considered for development as a candidate for the treatment of advanced FR-expressing human cancers.

Folate targeting enables durable and specific antitumor responses from a therapeutically null tubulysin B analogue.

The membrane-bound high-affinity folate receptor (FR) is highly expressed on a wide range of primary and metastatic human cancers, such as those originating in ovary, lung, breast, endometrium, kidney, and brain. Because folate-linked conjugates bind to and become internalized within FR-expressing cells (similar to that of free folic acid), we explored the possibility of using the folate ligand to target a potent, semisynthetic analogue of the microtubule inhibitor tubulysin B to FR-enriched tumors. When tested in vitro, a novel folate conjugate, herein referred to as EC0305, was found to specifically inhibit the growth of a panel of FR-positive cell lines (IC50 range, 1-10 nmol/L) in a dose-dependent manner, whereas cells lacking FR expression were unaffected. The potency of EC0305 was also confirmed against a human KB xenograft-nu/nu mouse cancer model. Here, a brief three times per week, 2-week regimen yielded remarkable antitumor activity (100% tumor-free animals) without causing significant weight loss or major organ tissue degeneration. In contrast, antitumor activity was completely abolished in EC0305-treated animals that were co-dosed with an excess of a nontoxic folate-containing analogue, thereby confirming that the antitumor effect of this agent was mediated by FRs. The advantage provided by folate conjugation was further proved by the untargeted free drug, which was found to be completely inactive at both tolerable and highly toxic dose levels. Collectively, these results show that this potent antiproliferative tubulysin compound can be specifically delivered to FR-positive tumors to provide substantial therapeutic benefit using well-tolerable dosing regimens.

Impact of high and low folate diets on tissue folate receptor levels and antitumor responses toward folate-drug conjugates.

Herein, we present a detailed analysis on the effects of feeding laboratory mice both high and low folic acid (folate)-containing diets as related to associated changes in serum and red blood cell (RBC) folate levels, tissue-derived folate receptor levels, and the ability of folate-drug conjugates to bind and effectuate activity against folate receptor (FR)-positive tumor xenografts. Our data show that serum and RBC folate concentrations sharply drop immediately after mice are switched to low folate diets; however, both parameters reach steady-state, "human-like" levels after 6 weeks. Interestingly, tissue-related folate binding capacities were also lowered during the dietary modulation period, whereas the net uptake of a radiolabeled folate conjugate was simultaneously increased 2.6- and 5-fold in FR-positive kidney and tumor tissue, respectively. Finally, the performances of several clinically and preclinically relevant folate-drug conjugates were evaluated against tumors in mice that were fed high or low folate diets. Except when administered at a dose level 6-fold less than that required to saturate endogenous FRs, no significant loss of antitumor activity was observed. From these findings, we conclude that lowering the dietary intake of folates in mice has little impact on the biological activity of repetitively dosed folate-targeted agents but that low folate diet regimens will reduce serum and RBC folate levels down to levels that more closely approximate the normal human ranges.

Preclinical antitumor activity of a novel folate-targeted dual drug conjugate.

We have designed a new type of tumor-targeted agent by tethering two different drug molecules, with distinct biological mechanisms of action, to the same ligand. This compound, named EC0225, represents the "first in class" multidrug, folate receptor (FR)-targeted agent to be disclosed. It was constructed with a single folate molecule, extended by a hydrophilic peptide-based spacer, which was in turn attached to mitomycin and Vinca alkaloid units via two separate disulfide-containing linkers. EC0225 produced potent, dose-responsive activity in vitro, and curative activity was observed against FR-positive syngeneic and xenograft tumors following the administration of well-tolerated dosing regimens. Multiple complete responses and cures were also noted when EC0225 was used to treat mice initially bearing tumors as large as 750 mm (3) in volume. Overall, EC0225's impressive preclinical activity allowed for its selection as a development candidate and for the start of Phase 1 clinical trials, which began in March of 2007, for the treatment of advanced malignancies.

Preclinical evaluation of EC145, a folate-vinca alkaloid conjugate.

We recently developed a new group of folate-conjugated Vinca alkaloids, one of which, EC145, emerged as a candidate for clinical development. Brief treatment of nude mice bearing approximately 100 mm(3) folate receptor-positive human xenografts led to complete response (CR) in 5/5 mice and cures (i.e., remission without a relapse for >90 days post-tumor implantation) in 4/5 mice. Multiple CRs and cures were also noted when EC145 was used to treat mice initially bearing tumors as large as 750 mm(3). Likewise, complete cures (5/5) resulted following the treatment of an aggressive folate receptor-positive J6456 lymphoma model. The activity of EC145 was not accompanied by noticeable weight loss or major organ tissue degeneration. Furthermore, no significant antitumor activity (0/5 CR) was observed in EC145-treated animals that were co-dosed with an excess of a benign folate ligand, thus demonstrating the target-specific activity of EC145. The enhanced therapeutic index due to folate conjugation was also evidenced by the fact that the unconjugated drug (desacetylvinblastine monohydrazide) was found to be completely inactive when administered at nontoxic dose levels and only marginally active when given at highly toxic dose levels. Subsequent dose regimen studies confirmed that EC145 given on a more frequent, qdx5 schedule resulted in the most effective antitumor response as compared with an equivalent total dose given on thrice- or single-injection-per-week schedule. Taken together, these studies show that EC145 has significant antiproliferative activity and tolerability, thus lending support to an ongoing phase 1 trial for the treatment of advanced malignancies.