Multicenter Phase II Trial of the PARP Inhibitor Olaparib in Recurrent IDH1- and IDH2-mutant Glioma

Purpose: Isocitrate dehydrogenase (IDH) 1 and IDH2 mutations (IDH1/2mt) are frequent in glioma. Preclinical studies suggest IDH1/2mts confer “BRCAness” phenotype, a vulnerability that can be targeted through PARP inhibition. To test this hypothesis, we conducted a multicenter study of olaparib monotherapy in patients with IDH1/2mt gliomas. Methods: Patients with recurrent, contrast-enhancing IDH1/2mt gliomas were enrolled in a two-step phase II trial; the primary endpoint was overall response rate per Response Assessment in Neuro-Oncology (RANO) criteria. Olaparib 300 mg orally twice daily was given. Results: A total of 15 evaluable patients were enrolled. Histology was astrocytoma (N = 12) and oligodendroglioma (N = 3). Most toxicities were grade 1 or 2. Best response was stable disease (SD) in 9 (60%) patients. Median progression-free survival (PFS) was 3.63 months and median overall survival was 20.7 months. For patients with SD, median PFS was 5.53 months; 4 patients had SD for >6 months. Among patients with best response progressive disease (N = 6), 5 had grade 4 tumor and 4 had known CDKN2A alteration. PFS was 5.23 months for grades 2 or 3 tumors (N = 10) versus 1.8 months for grade 4 (N = 5; P = 0.0013). Conclusion: The study did not meet the prespecified response-based activity threshold for moving to step 2. However, prolonged SD was observed in patients with grades 2 and 3 histologies, suggesting olaparib monotherapy could be of clinical benefit in select populations. Grade 4 tumors per 2021 World Health Organization classification defined by histology or CDKN2A alteration derived no benefit from this drug, highlighting the usefulness of this classification for future patient stratification and trial design. Significance: A single-arm phase II trial of olaparib in IDH-mutant glioma demonstrated clinically significant prolonged SD for select patients with grade 2/3 disease, suggesting potential benefit of olaparib in IDH-mutant gliomas.


Introduction
Gliomas represent a heterogenous group of diseases that account for most primary brain tumors (1). The discovery of isocitrate dehydrogenase (IDH)  and IDH mutations (IDH/mt) in a subset of adult-type diffuse gliomas (2) and the observed improved prognosis associated with this phenotype (3) have led to the incorporation of this disease-defining molecular feature into the World Health Organization (WHO) classification of brain tumors (4). IDHmts occur in more than 70% of what was formerly termed "low-grade" gliomas and up to In general, first-line treatment for gliomas includes maximal safe resection which is often limited by tumor extent and location, adjuvant radiation, and alkylator-based chemotherapy. Chemotherapy options include temozolomide or combination of procarbazine, lomustine, and vincristine (7)(8)(9)(10)(11). While a better prognosis is observed in oligodendrogliomas, the disease invariably recurs.
Hence novel therapy options are clearly needed.
In addition to its prognostic significance, IDH/mt is also a promising therapeutic target in the treatment of glioma. The normal function of IDH is to catalyze conversion of isocitrate to alpha-ketoglutarate in the citric acid cycle (12). IDH/mts induce aberrant formation of the oncometabolite 2hydroxyglutarate (2HG), which is implicated in cancer progression through its inhibitory effect on alpha-ketoglutarate-dependent dioxygenases (13,14). IDH has been successfully targeted in other malignancies that carry IDHmt. Currently, two drugs have been approved by the FDA: ivosidenib, approved for IDHmt acute myeloid leukemia (AML) and cholangiocarcinoma, and enasidenib, approved in IDHmt AML (15)(16)(17). Trials investigating ivosidenib and vorasidenib in IDHmt gliomas have been published recently (18,19) and are also ongoing (20,21).
Preclinically, IDH/mts induce a homologous recombination defect which renders tumor cells sensitive to PARP inhibitors (PARPi; ref. 22). This "BRCAness" phenotype, with sensitivity to PARPi, is due to a single alphaketoglutarate-dependent dioxygenase targeted by 2HG, KDM4A, which mediates homologous recombination (22). IDH-dependent PARPi sensitivity has been shown in culture in patient-derived glioma cells as well as genetically matched tumor xenografts in vivo (22). This "BRCAness" was significant in IDH/mt cells and approached a 50-fold difference compared with IDH wildtype cells when exposed to olaparib (22). Olaparib, an orally bioavailable PARPi, has central nervous system (CNS) penetrance (23,24) and is currently FDA approved for the treatment of several BRCA-mutated tumors (25). We report the results of a multicenter prospective phase II study investigating the role of olaparib monotherapy in recurrent or progressive IDH/mt gliomas.

Patient Selection
Eligible patients had recurrent or transformed glioma that progressed despite standard therapy or for which no effective standard therapy existed with evidence of an IDH/mt associated with neomorphic activity of encoded proteins. On the basis of archived histology, tumors of all grades were eligible, but the presence of tumor contrast enhancement on T1 post-gadolinium MRIs performed prior to enrollment was required for all patients, as a surrogate marker of progression to a high-grade histology. Specific criteria for patient eligibility, tumor classification, and progression (4) can be found in Supplementary Data S1. The representativeness of study participants is discussed in Supplementary   Table S1.

Study Treatment and Design
The primary objective of this trial was to estimate the overall response rate (ORR) of olaparib in patients with IDH inhibitor naïve IDH/mt glioma.
Secondary objectives were to estimate the distribution of progression-free survival (PFS), the overall survival (OS) and the duration of response in this population. Safety and tolerability of olaparib were also evaluated. Treatment cycles were 28 days. Patients were administered olaparib in tablet form orally 300 mg twice daily continually, irrespective of food intake. No premedications were required. Toxicities were graded using the NCI Common Terminology Criteria for Adverse Events version 5.0. Retreatment criteria can be found in Supplementary Data S2. Patients were monitored for disease status using Response Assessment in Neuro-Oncology (RANO) criteria with MRI performed every 8 weeks. Because of preliminary findings suggesting prolonged stable disease (SD) in some patients, an independent review was commissioned. Results were compared with those reported by local investigator. Any discordance was reported and investigated.
Written informed consent was obtained from enrolled patients. Institutional Review Board approvals of the protocol and consent forms were obtained from all sites. Protocol design and conduct complied with all applicable regulations, guidance, and local policies. The study was conducted in accordance with the Declaration of Helsinki. The trial was conducted under an NCI-sponsored Investigational New Drug application (www.ClinicalTrials.gov: NCT03212274).
Olaparib was supplied by the Division of Cancer Treatment and Diagnosis of the NCI.

Statistical Methods
The power analysis and sample size estimation were completed using the Bayesian adaptive trial design method. We modeled the probability of ORR using Beta distributions. The prior distribution is Beta(0.1, 0.9), which is fairly pessimistic; the expected value is 0.1, reflecting the response rate based on previous experiences. We estimated the ORR using the posterior distribution and reported its median and 90% credible interval (see protocol for details). It was predetermined that the trial would be terminated if there was a lack of response, that is, the probability of the ORR being greater than 0.1 was 50% or less. Because the ORR in the trial was zero, the trial was terminated. The SD rate and the progressive disease (PD) rate were reported, and the 90% credible intervals were estimated using the posterior distribution. SD was defined as lack of progression at first restaging after two cycles.
For secondary objectives, the median PFS and OS were estimated using Kaplan-Meier estimate with 95% confidence intervals (CI). The CIs were based on Greenwood formula for variance. Possible risk factors for PFS and OS of WHO grade and CDKNA alteration were compared using the log-rank test.
For the secondary objectives, all tests were two sided. Statistical analyses were performed using the R 4.1.1 software.

Data Availability
The data generated in this study are available upon request from the corresponding author.

Patient Demographics
Sixteen patients were enrolled. Fifteen patients were evaluable for response and toxicity. One patient was found to be ineligible after signing consent ( Fig. 1). Gliomas were further divided according to WHO 2021 grading system by the following subtypes: grades 2 and 3 astrocytomas (n = 7), grade 4 astrocytoma, formerly known as gliblastoma (n = 5), and grades 2 and 3 oligodendrogliomas (n = 3) (Table 1). IDHmt was confirmed for each patient, either through prior documentation or testing during enrollment period. Tumors

AACRJournals.org
Cancer Res Commun; 3(2) February 2023 from 13 of 15 patients were found to have mutations that led to the R132H neomorphic phenotype in the IDH gene (one tumor also contained a variant mutation of uncertain significance in IDH) and 2 of 15 patients had tumors which harbored neomorphic mutations in the IDH gene (R172G and R172K; Table 2). CDKNA copy-number loss was documented in 5 patients (Table 2). Patient demographics and prior treatments are summarized in Table 1. All patients had evidence of progression at enrollment.

Toxicity
There were no unexpected toxicities reported and most toxicities were grade 1 or 2. The most common toxicities were nausea (67%) and fatigue (47%; Table 3). One patient with a history of ulcerative colitis reported grade 3 diarrhea that resulted in a dose reduction to 250 mg twice daily after the second cycle. The patient was taken off study during cycle 5 due to continued drugrelated diarrhea. One patient developed grade 3 hypertension that was possibly attributed to olaparib. Onset was after patient stopped treatment at cycle 12 and subsequently improved at follow-up visit. Finally, grade 3 lymphopenia was reported in 1 patient.
Reevaluation of imaging of all patients by a blinded NCI-designated neuroradiologist was performed. Two discordant results were identified: one patient's tumor had progressed after six cycles of treatment but was kept on for four additional cycles by the enrolling site. The corrected months of progression-free disease was reported in the above data. The second patient with discordant results had been taken off treatment due to PD after five cycles by their treating physician. However, reassessment of their scans demonstrated SD.

Discussion
Olaparib monotherapy was well tolerated in patients with recurrent or progressive contrast-enhancing IDH/mt glioma but did not meet prespecified response-based activity criteria for moving to stage 2 of the trial. However, clinically significant prolonged SD was observed in a relatively large proportion of patients, particularly those with WHO grades 2 and 3 histologies, suggesting  PARPi have also been seen in tumors with DNA repair alterations other than BRCA mutations (29,30). Despite preclinical susceptibility of IDH/mt tumors to PARPi, our trial did not meet the prespecified endpoint of ORR to unequivocally demonstrate biological activity deriving from the "BRCAness" effects associated with 2-HG. However, the PFS data and disease stability observed in a relatively large proportion of patients (60%) raises the question of whether PARPi could still be of clinical benefit in these patients, in the absence of measurable tumor shrinkage. While our findings may suggest potential benefit in patients with lower grade histologies or at earlier stages of disease progression, further investigation is required as such phenotypes are inherently less aggressive. Other trials have studied PARPi in patients with gliomas, but their inclusion criteria likely excluded most patients with IDH/mt and/or other homologous recombination defects (31,32). In our trial, selection of patients based on contrast enhancement was an attempt to focus on the patients with IDHmt tumors of a higher grade or more aggressive behavior. However, the molecular heterogeneity of patients included in our study, patients' enrollment at varying stages in their tumor's natural history, the small number of patients with oligodendroglioma, and differences in timing of collection of molecular information (at diagnosis vs. at time of recurrence) limit the conclusions that can be drawn from our data. Regardless, it is clear from the results of this trial that single-agent olaparib is not a sufficient treatment for grade 4 gliomas.
Recently, preliminary results of the OLAGLI trial investigating olaparib in 35 patients with recurrent IDH/mt high-grade contrast-enhancing gliomas were presented at American Society of Clinical Oncology 2021 (33). That study found a 6-month PFS rate of 31%, which did not meet prespecified threshold for significance, but 2 patients (5%) had a PR and 14 patients (40%) achieved SD. The median duration of response in the combined PR and SD patients was 9 months. The median OS was 15.9 months (33). Similar to our study, these preliminary results suggest that there may be patients who experience clinically meaningful benefit from single-agent PARPi through disease stability, although olaparib's activity as a single agent as measured by radiographic responses and tumor shrinkage is modest.

Adverse event Total number of patients with event N (%) Grade 1 (%) Grade 2 (%) Grade 3 (%)
Nausea 10 (67%) 9 (90%) 1 (10%) -  An important concern in the development of novel therapies for CNS tumors is the ability of novel agents to penetrate through areas of intact blood-brain barrier to address infiltrative disease. CNS penetration of olaparib in humans has been shown in a phase I study combining temozolomide with olaparib in patients with relapsed glioblastoma; olaparib, given prior to surgery, was detected in 71 of 71 tumor core specimens from 27 patients (34). Detected drug levels were greater than 100 nmol/L, the concentration determined by in vitro experiments to be needed for maximal PARP inhibition (35). Trials are ongoing investigating novel PARPis with potential for even greater brain penetration (NCT00687765, NCT03914742).
Identifying mutations that further define distinct subgroups within IDHmt glioma for updated grading and selection of patients who may benefit from PARPi remains of high interest. Homozygous deletion of CDKNA has been noted to be an adverse prognostic factor associated with worse PFS and OS in patients with IDH/mt disease (36,37) and per the 2021 WHO classification of tumors confers grade 4 status (6). In our study, 4 of 5 patients with known CDKNA alteration progressed on first restaging. Our findings therefore seem to support the inclusion of CDKNA status as a grade-defining molecular finding for the grading of gliomas, as acknowledged in this new classification of gliomas by the WHO. A limitation in our observation is that CDKNA copynumber status was assessed by next-generation sequencing (NGS) performed locally, and the distinction between homozygous and heterozygous deletion was not uniformly provided. While in IDH wildtype glioblastoma CDKNA deletions are nearly universally homozygous, we acknowledge that in some patients with IDH/mt gliomas, the CDKNA deletion as determined by NGS may not be homozygous. Therefore, it is possible, but unlikely, that the 2 patients with poor outcomes that were reclassified as grade 4 per WHO 2021 may not meet criteria for such, should other methods for CDKNA evaluation be utilized.
In addition to PARPi, a significant focus of research in IDH/mt gliomas is on direct inhibition of IDHmt proteins. In a phase I trial, 30 patients with recurrent or progressive IDH/mt enhancing glioma received vorasidenib, an oral, brain-penetrant inhibitor of IDH/mt. ORR was 0% but 56.7% of patients achieved SD. The median PFS was 3.6 months, similar to our trial (19). A window-of-opportunity trial of vorasidenib and the IDH inhibitor ivosidenib randomized patients with recurrent nonenhancing WHO 2016 grade 2 or 3 IDHmt glioma who were undergoing craniotomy to receive either ivosidenib, vorasidenib, or no treatment for 4 weeks preoperatively. Postoperatively, they were randomized to ivosidenib or vorasidenib. Both drugs were found to be CNS penetrant (brain:plasma ratio 0.16 for ivosidenib and 2.4 for vorasidenib) and both lowered 2HG levels compared with untreated samples (21). ORR was 30.8%. Switching to a focus on earlier stage disease, the phase III INDIGO trial is randomizing IDH/mt residual or recurrent grade 2, nonenhancing glioma to receive vorasidenib or placebo, with primary endpoint of PFS (38).
There are numerous future study directions in this population. Trials investigating timing of IDH-targeted therapy with IDH inhibitors or PARPi in the AACRJournals.org Cancer Res Commun; 3(2) February 2023 first-line (NCT03581292) or recurrent/relapsed setting and timing with respect to surgery will be important. An ongoing study is investigating the effect of niraparib prior to surgery in IDHmt glioma (NCT05076513). Studies that compare responses in patients with contrast-enhancing and non-contrastenhancing tumors will also provide information on which populations are most likely to benefit. Outcomes of treatment with PARPi in patients who have received prior treatment with IDH inhibitors are currently being investigated in a phase II clinical trial (NCT03212274). Finally, it is possible that PARPi may require combination with additional therapies to improve activity and induce tumor shrinkage. A single-center retrospective study reported the results of patients with glioma, some with IDH/2mt, who were treated with combination olaparib and temozolomide. Four of the 8 patients with grade 2 or 3 IDHmt disease had complete or partial response. Consistent with our findings, no responses were seen in patients with grade 4 IDHmt glioma. This trial suggests promise of olaparib in combination but is limited by its retrospective design (39). Other ongoing trials investigating combination therapy include PARPi + PD-L1 inhibitor (NCT03991832), PARPi + temozolomide (NCT02152982, NCT04552977, NCT01390571, NCT04910022, NCT03581292, NCT03749187, NCT03914742, NCT03212742), and PARPi + temozolomide + radiotherapy (NCT04614909). Taken together, this body of work will establish whether PARPi will eventually play a meaningful role in the treatment of gliomas.