Histone deacetylases (HDACs) are commonly overexpressed in many types of cancers (1, 2), an observation that has led to the emergence of cancer therapies targeting HDACs (3). HDAC inhibitors (HDACi) have been shown to reestablish normal histone acetylation patterns and demonstrate antitumorigenic effects, including cell death, cell cycle arrest, suppression of angiogenesis, and immunomodulation (1–6). However, monotherapy with pan-HDACi has demonstrated limited clinical benefit (7–9). Furthermore, development of pan-HDACi has been limited by high-grade toxicities, as reported in the PANORAMA1 study in myeloma (10).

Citarinostat (CC-96241; previously ACY-241) is an oral selective HDAC6 inhibitor that exhibits potent biochemical inhibition of HDAC6 with 13- to 18-fold reduced potency against the nuclear class I HDACs, including HDAC1, HDAC2, and HDAC3 (11). In a preclinical study using A2780 ovarian cancer cells, treatment with citarinostat (300 nM) for 24 hours increased α-tubulin acetylation, which is consistent with tubulin deacetylase HDAC6 inhibition. In addition, acetylation of histones (targeted by class I HDACs) was also increased but only at citarinostat doses exceeding 1 μM. Collectively, these preclinical data suggest that low-dose citarinostat may selectively inhibit HDAC6; however, increased citarinostat dosing may also inhibit class I HDAC isozymes.

HDAC6 inhibition may increase microtubule stability and has been shown to mechanistically enhance the tubule-stabilizing action of taxane chemotherapy, leading to increased cell death in preclinical models (12, 13). For example, pretreatment of anaplastic thyroid carcinoma cells with valproic acid, which inhibits HDAC6, improved the anticancer activity of paclitaxel (13). Similarly, class I HDAC inhibition may also augment the antitumor activity of taxanes; romidepsin, an RPD3-like class I HDACi, was shown to have a synergistic effect when combined with paclitaxel in a preclinical model of breast cancer (12, 14). In a phase II trial of patients with previously untreated stage IIIB or IV non-small cell lung cancer, vorinostat (a class I and II HDACi) plus carboplatin and paclitaxel resulted in a significantly higher response rate as well as a trend in improved progression-free survival and overall survival compared with carboplatin plus paclitaxel alone (15). However, the rate of grade 4 thrombocytopenia was notably higher with vorinostat plus chemotherapy vs chemotherapy alone. In a phase 1a/b first-in-human study in patients with relapsed or refractory multiple myeloma, citarinostat alone or in combination with pomalidomide and dexamethasone demonstrated promising safety and tolerability (16).

A preclinical study demonstrated that citarinostat (or the structurally related ricolinostat, the liquid formulation of citarinostat) in combination with paclitaxel suppressed the growth of multiple solid tumor lineages, including pancreatic, ovarian, and breast cancer cell lines, and significantly reduced the tumor volume in ovarian and pancreatic xenograft models compared with either agent alone (11). These results prompted us to conduct a phase Ib study designed to determine the safety, pharmacokinetics, pharmacodynamics, and preliminary antitumor activity of citarinostat plus paclitaxel in patients previously treated with standard regimens for advanced solid tumors.

This study was approved by the institutional review board at each participating site. Study monitoring was conducted in compliance with the International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use and current Good Clinical Practice guidelines and the general ethical principles of the Declaration of Helsinki. Written informed consent was obtained from all patients before study entry. This trial is registered on ClinicalTrials.gov (NCT02551185).

In this phase Ib study, no unexpected toxicities or DLTs were observed with citarinostat in combination with paclitaxel in patients with heavily pretreated advanced solid tumors. As a result, the MTD was not reached, and the planned maximum administered dose showed no benefit over the next lower dose: at the 480- and 360-mg doses, 1 and 2 confirmed PRs, respectively, were observed. Citarinostat exposure doubled from the 180- to 360-mg doses but did not increase substantially with the 480-mg dose; with the 360- and 480-mg doses, exposure was generally similar from cycle 1 day 1 to day 15. One potential cause for the lack of exposure increase at the 480-mg dose could be saturable absorption, but this was not investigated further. Peak acetylation of histone and tubulin occurred at the 360-mg dose. On the basis of the safety, pharmacokinetic, and pharmacodynamic results, the RP2D of citarinostat was determined to be 360 mg once daily in combination with paclitaxel 80 mg/m² qw 3/4. The tolerability of this dose was confirmed in an additional cohort of patients that included only those with disease that had previously progressed on a taxane. Overall, many patients were previously treated with taxanes, and that previous treatment did not preclude clinical benefit with this combination. In general, paclitaxel exposure decreased with increased doses of citarinostat. This finding is consistent with the observation that paclitaxel may be altered by inducers of cytochrome P450 (CYP) 2C8 and/or CYP3A4, and citarinostat is a moderate inducer of CYP1A2 and CYP3A4 (unpublished results).

Historically, the toxicity profile of pan-HDACi has been prohibitive and has limited their use in combination chemotherapy regimens. Several pan-HDACi approved for hematologic indications—including vorinostat, panobinostat, and romidepsin—have shown evidence of cardiac toxicity, specifically QT prolongation (18), which increases the risk of malignant cardiac arrhythmias and sudden cardiac death. As a selective HDAC6 inhibitor with less extensive inhibition of class I HDACs, citarinostat demonstrated an acceptable safety profile, with no reports of cardiac toxicity or QT prolongation. More hematologic toxicity was observed at the 360-mg than the 480-mg dose, but the pharmacokinetics does not suggest that this is a direct drug-mediated event. Patients in the 360-mg group more frequently had heavily pretreated cancers (such as ovarian cancer), which place patients at higher risk of hematologic toxicities, than patients in the 480-mg group (19). Pan-HDACi and chemotherapy combinations such as vorinostat, paclitaxel, and carboplatin have been associated with noted thrombocytopenia (15), but the only hematologic toxicities observed with citarinostat and paclitaxel were neutropenia, leukopenia, and anemia. Altered hematotoxicity with citarinostat and paclitaxel could be due to the HDAC6-specific inhibition of citarinostat or the citarinostat-induced changes to paclitaxel-based pharmacokinetics. Not surprisingly, most peripheral neuropathy events were likely due to paclitaxel treatment, which is known to be associated with this AE (20). The incidence of grade 3/4 peripheral neuropathy reported in this study (25%) was within the range of that reported previously with paclitaxel alone at various doses and schedules (2%–33%) (20), including 90 mg/m² qw 3/4 (18%) (21). One patient discontinued treatment due to peripheral sensory neuropathy associated with paclitaxel. Two patients experienced ≥ 1 neuropathy-related TEAE associated with paclitaxel, leading to dose reduction. Collectively, these findings support the tolerability of the regimen.

The pharmacodynamic data revealed increased tubulin acetylation over time with citarinostat 360 and 480 mg as well as histone acetylation with citarinostat 360 mg (and less pronounced but noticeable histone acetylation with 480 mg). Tubulin acetylation levels increased in response to paclitaxel before citarinostat administration and subsequently increased further with citarinostat but only at the 360- or 480-mg dose level. Histone acetylation with citarinostat has been observed previously, albeit with higher drug concentrations than those required to increase tubulin acetylation (11). Although the sample numbers in our study were small, our findings suggest that citarinostat at the 360-mg dose may exhibit activity against both HDAC6 and class I HDACs and raise the possibility that a dose between 180 and 360 mg may be required to demonstrate the relative selectivity of the agent for HDAC6.

Interpretations of this phase Ib study of citarinostat plus paclitaxel are limited due to the small size of each cohort and lack of a comparator arm. The MTD of citarinostat was not reached, and an RP2D of 360 mg daily citarinostat in combination with paclitaxel 80 mg/m² qw 3/4 was established. The safety profile and potential antitumor activity of citarinostat plus paclitaxel in patients with heavily pretreated advanced solid tumors reported in this study are encouraging; however, further studies are needed to evaluate whether there is clear evidence of efficacy with the addition of citarinostat or with this combination.

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.

The studies involving human participants were reviewed and approved by the International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use and current Good Clinical Practice guidelines and the general ethical principles of the Declaration of Helsinki. This study was approved by the institutional review board at each participating site. The patients/participants provided their written informed consent to participate in this study.

MG: Conceptualization, Formal analysis, Investigation, Methodology, Project administration, Resources, Supervision, Writing – original draft, Writing – review and editing. GS: Conceptualization, Formal analysis, Investigation, Methodology, Project administration, Resources, Supervision, Writing – original draft, Writing – review and editing. JS: Conceptualization, Formal analysis, Investigation, Methodology, Project administration, Resources, Supervision, Writing – original draft, Writing – review and editing. DJ: Conceptualization, Formal analysis, Investigation, Methodology, Project administration, Resources, Supervision, Writing – original draft, Writing – review and editing. BL: Conceptualization, Formal analysis, Methodology, Project administration, Resources, Supervision, Validation, Writing – original draft, Writing – review and editing. AZ: Data curation, Formal analysis, Validation, Writing – original draft, Writing – review and editing. JC: Conceptualization, Data curation, Formal analysis, Methodology, Resources, Supervision, Validation, Writing – original draft, Writing – review and editing. YLB: Conceptualization, Data curation, Formal analysis, Methodology, Project administration, Resources, Supervision, Validation, Writing – original draft, Writing – review and editing. CD: Conceptualization, Data curation, Formal analysis, Methodology, Resources, Supervision, Validation, Writing – original draft, Writing – review and editing. RH: Conceptualization, Formal analysis, Investigation, Methodology, Project administration, Resources, Supervision, Writing – original draft, Writing – review and editing. All authors contributed to the article and approved the submitted version.

Funding for this research was provided by Celgene, a Bristol-Myers Squibb Company, Summit, NJ, and an NCI Cancer Center Support grant (#P30 CA054174) to the Institute for Drug Development, Cancer Therapy and Research Center at The University of Texas Health Science Center at San Antonio, San Antonio, TX.

MG: employment, Arizona Center for Cancer Care; stock, Medelis, Care Mission USA; honoraria, Deciphera; consulting/advisory role, Deciphera, RedHill Biopharma, Salarius; research funding, AbbVie, Acetylon, Celgene, a Bristol-Myers Squibb Company, Celldex, Corcept, Calithera, Deciphera, Eisai, Endocyte, Eli Lilly, Five Prime, Genentech, Plexxikon, Pfizer, MedImmune, Merck, OncoMed. GS: research funding, Eli Lilly, Merck KGaA/EMD Serono, Merck, Sierra Oncology; consulting/advisory role, Almac, Angiex, Astex, Bayer, Bicycle Therapeutics, Cybrexa Therapeutics, Daiichi Sankyo, Eli Lilly, Fusion Pharmaceuticals, G1 Therapeutics, Ipsen, Merck KGaA/EMD Serono, Pfizer, Roche, Sierra Oncology. DJ: consulting/advisory role, Eisai, EMD Serono, Novartis. BL, YLB, and CD: employment, Bristol Myers Squibb; stock, Bristol Myers Squibb. AZ: employment, Celgene, a Bristol-Myers Squibb Company; stock, Bristol Myers Squibb. JC: employment, Bristol Myers Squibb; stock, Bristol Myers Squibb; research funding, Celgene, a Bristol-Myers Squibb Company; travel, Celgene, a Bristol-Myers Squibb Company. RH: consulting/advisory, Amgen, GlaxoSmithKline; research funding, Abbisko, AbbVie, Actuate, Alkermes, Amgen, AstraZeneca, Bayer, Bristol Myers Squibb, Boston Biomedical, Calithera, Celgene, a Bristol-Myers Squibb Company, FujiFilm, Genmab, GlaxoSmithKline, Infinity, InhibRx, Merck, Mersana, Meryx, Nektar, Pfizer, Puma, RAPT Therapeutics, Regeneron, Rgenix, Sanofi, Seattle Genetics, Sutro, Takeda, Xencor. The authors declare that this study received funding from Celgene Research S.L.U. The funder was involved in the study design, collection, analysis, interpretation of data, and funded the writing of this article.

The remaining author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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