In studying the last 120 clinical trials in GBM it became apparent that all had robust preclinical data showing benefit of the given drug in xenograft GBM models yet none translated into clinical benefit in phase 3 trials.
We designed CUSP9v3 to address the following attributes of GBM that we believe are the cause of failure of the last 120 clinical trials reported on PubMed.org:
- Spatial heterogeneity of growth drive dependency;
- Temporal heterogeneity of these;
- Existence of mutually supporting cell communities;
- Compensatory tumor responses to irradiation, chemotherapy by A. vertical clonal selection; B. horizontal sharing of resistance mechanisms via exosomes, etc.;
- Existence of multiple cross-covering growth driving signaling pathways;
- Metabolic flexibility in that if one energy source becomes inhibited,reliance is shifted to another;
- Glioblastoma’s enlistment of normal physiologic somatic systems that are normally functioning but pathologically engaged.
- Entrance into a dormant state.
Our conclusion is that a polypharmaceutical approach is warranted until and unless a “silver bullet” is found.
CUSP9v3 is well-enough tolerated to be started in outpatients and to be fully introduced over a shorter time period than the 35 days that we used. In this pilot trial in recurrent or progressive GBM, we found that 9 carefully selected non-oncological repurposed drugs together with twice daily 20 mg/m2 BSA temozolomide was safe and generally well-tolerated if individual dose adjustments were performed.
Other trials in pediatric and adult high-grade glioma had reported the safety of various multi-agent regimens combining chemotherapy with repurposed drugs, using a range of 4-7 agents [23-25]. Here we show that it is possible to combine 9 repurposed drugs given careful evaluation of potential drug-drug interactions and cumulative toxicity. Knowing that many non-oncological drugs target pathways relevant to GBM, precision oncology approaches could expand their armamentarium by evaluating non-cancer drugs and combining them with classical cancer drugs [26, 27]. Such was recently reported in a trial in diffuse intrinsic pontine glioma .
Strategies targeting cell membrane marker-defined GBM cells may be limited in that growth drive dependency is an “ever shifting target” in GBM [29, 30]. CUSP9v3 is consciously intended as a biomarker-independent approach.
In the small group of participants studied and reported here, longer-than-expected PFS and OS were observed also in patients whose tumors had unfavorable molecular profiles (i.e., absence of isocitrate dehydrogenase [IDH] mutation and/or of O6-methylguanine-DNA methyltransferase [MGMT] promoter hypermethylation).
While in our trial the cumulative doses of minocycline, sertraline and auranofin each strongly correlated with PFS and OS, the most effective antiproliferative drugs against GBM cells in vitro were ritonavir, disulfiram and auranofin when used in clinically relevant concentrations (Halatsch et al., unpublished data), with auranofin notably listed for both settings.
During the protocol development for this trial, a hierarchical drug list had been developed (based on AE information contained in each drug’s summary of product characteristics) that correlated AEs to ranked sequences of drugs to be halved in dose or paused until pre-specified lower CTCAE grades were reached. If that was not the case within 3 days, the next drug on the hierarchical list was halved in dose or paused etc. This strategy proved successful in managing AEs.
Noteworthy, the three AEs most frequently observed in this trial were nausea, headache and fatigue. While a causal relationship between the CUSP9v3 drugs and these cardinal AEs cannot be excluded, these symptoms may also be caused by the underlying disease itself and/or its primary treatment, temozolomide.
In terms of the strong Spearman rank correlation between the applied scheduled cumulative CUSP9v3 total dose and PFS or OS, respectively (Supplement 6), there are several paths of putative causality. In most cases of pronounced CUSP9v3 cumulative dose reductions, tumor progression and the associated decrease of functional status impaired the ability of oral drug intake; in some patients with good functional status, however, specific drug side effects may have indirectly caused tumor progression via (albeit targeted) dose reductions and/or drug pausing. Those patients may benefit from an adaptive trial protocol with a pre-specified “shadow cocktail” designed to selectively replace individually intolerable drugs before or even upon tumor progression, with participants conditionally remaining on-study (in a future trial).
Lastly, a smaller fraction of patients – despite having a good functional status – may find simultaneous oral intake of a multiplicity of separate drugs genuinely challenging. For this latter group, the so-called polypill approach with possible integration of all CUSP9v3 drugs into one 3D-printed polypill (with flexible dosing options) may prove advantageous .
While the trial was not designed to assess efficacy of the CUSP9v3 regimen, we observed that 5 patients progressed quickly, dying within a range of 1.5-7 months. The 5 other patients did well on treatment, all 5 having a PFS of 12 or more months (range 12-29 months at the time of data lock).
Such pattern of failure, as seen in the Kaplan-Meier survival curve (Fig. 3) is distinctly unusual in GBM trials. In recurrent GBM, single-agent trials have generally reported PFS at 6 months of 20-30% [32, 33]. The rate of patients being alive and progression-free at 6 months has been suggested as an appropriate surrogate end point for predicting OS .
Regarding the correlation between cumulative CUSP9v3 total dose and PFS and OS, possible CUSP9v3 efficacy cannot be causally distinguished from longer survival simply enabling longer treatment duration.
Because of the small number of patients, the observed dichotomy of response is difficult to interpret. One hypothesis is that some or all of the 5 patients with a long PFS had pseudo-progression that erroneously led to inclusion in the study, although a 12-month period of stability would still not be common. The trial did not require histopathological confirmation of recurrence prior to study entry. Therefore, despite radiological judgement and reasonable time periods between the completion of radio- and chemotherapy on the one hand and the beginning of the study on the other hand (12 and 4 weeks, respectively), patients with a favorable course could have had pseudo-progression upon starting CUSP9v3. This issue of pseudo-progression is inherent to non-randomized trials in the recurrent setting.
Another hypothesis is that we are seeing a differential response according to pre-study lesion expansion rate, which would indicate that CUSP9v3 may be particularly effective in patients with slower proliferating tumors and/or lower tumor burden, suggesting that this regimen may have a role in a prophylactic maintenance setting after first-line treatment. The data presented in Fig. 4 would strongly support this notion. While a 12-month PFS of 50% intuitively appears discrepant with a median PFS of 3.0 months, this apparent discrepancy directly results from the pronounced dichotomy of response to CUSP9v3 that was observed in this trial.
A further note of caution in interpreting our results are the relatively favorable prognosis attributes of the patients with recurrent GBM recruited for this pilot study. Of the 5 study patients still alive at 12 months, 3 had tumors with MGMT promoter hypermethylation, one of whom was also IDH-mutated. The 2 other patients alive at 12 months had tumors without MGMT promoter hypermethylation but were 31 years of age or under upon study inclusion. These favorable prognostic factors could contribute to longer-term survival of these patients.
Drug repurposing represents a large source of therapeutic options in cancer . In GBM in particular, notably 76 repurposed drugs were reported as potentially useful in treating GBM . The selection of the 9 drugs to be included in CUSP9v3 was a long iterative process within a conceptual framework which considered the specific and relevant preclinical, pharmacological, and empirical features of each drug in addition to the 5 criteria listed in the Introduction. It should not be assumed that combining other repurposed non-cancer drugs will automatically yield similar results; other regimens may prove more or less toxic or more or less effective.
In the common aggressive cancers, and especially in GBM, phenotypic spatial and temporal heterogeneity, in both stem and non-stem subsets, is a dynamic process responding to treatment interventions and driven further and faster by hypoxia [30, 36-39]. GBM may be considered a collection of mutually interacting, mutually supporting cellular subpopulations demanding the use of a multi-drug combination to achieve prolonged treatment response. As in CUSP9v3 and in Palmer et al. we advocate “the 50-year old hypothesis that a curative cancer therapy can be constructed on the basis of independently effective drugs having non-overlapping mechanisms of resistance, without synergistic interaction, which has immediate significance for the design of new drug combinations” .