Adjuvant Sirolimus Does Not Improve Outcome in Pet Dogs Receiving Standard-of-Care Therapy for Appendicular Osteosarcoma: A Prospective, Randomized Trial of 324 Dogs

Abstract

Purpose: The mTOR pathway has been identified as a key nutrient signaling hub that participates in metastatic progression of high-grade osteosarcoma. Inhibition of mTOR signaling is biologically achievable with sirolimus, and might slow the outgrowth of distant metastases. In this study, pet dogs with appendicular osteosarcoma were leveraged as high-value biologic models for pediatric osteosarcoma, to assess mTOR inhibition as a therapeutic strategy for attenuating metastatic disease progression.

Patients and methods: A total of 324 pet dogs diagnosed with treatment-naïve appendicular osteosarcoma were randomized into a two-arm, multicenter, parallel superiority trial whereby dogs received amputation of the affected limb, followed by adjuvant carboplatin chemotherapy ± oral sirolimus therapy. The primary outcome measure was disease-free interval (DFI), as assessed by serial physical and radiologic detection of emergent macroscopic metastases; secondary outcomes included overall 1- and 2-year survival rates, and sirolimus pharmacokinetic variables and their correlative relationship to adverse events and clinical outcomes.

Results: There was no significant difference in the median DFI or overall survival between the two arms of this trial; the median DFI and survival for standard-of-care (SOC; defined as amputation and carboplatin therapy) dogs was 180 days [95% confidence interval (CI), 144-237] and 282 days (95% CI, 224-383) and for SOC + sirolimus dogs, it was 204 days (95% CI, 157-217) and 280 days (95% CI, 252-332), respectively.

Conclusions: In a population of pet dogs nongenomically segmented for predicted mTOR inhibition response, sequentially administered adjuvant sirolimus, although well tolerated when added to a backbone of therapy, did not extend DFI or survival in dogs with appendicular osteosarcoma.

Figure 1.

Overview of schema (A) and schedule of study procedures (B) for canine OS patients enrolled in the Standard of Care (SOC) vs. SOC + sirolimus (SOC + S) clinical trials. CXR = 3-view thoracic (chest) radiographic assessment. PBMC = Peripheral Blood Mononuclear Cells. PE = Physical Exam. PD = Progressive Disease. PK = pharmacokinetic assessment of drug levels.

Figure 2:

Events after enrollment are captured as dog were randomized to either Standard of Care (SOC) or SOC + sirolimus (SOC + S). Dogs that were removed from study prior to completing carboplatin chemotherapy were done so through their owners’ wishes. Sx = surgery.

Figure 3.

(a.) Red dots depict the sirolimus blood levels that were measured in dogs receiving 0.1 mg/kg orally on a Monday through Thursday basis within the SOC + S trial. These data were generated from whole blood of dogs (n = 61) on Cycle 1 Day 11 and Cycle 1 Day 25), and are overlaid with the simulated Area under the Curve (AUC) that was predicted from a simulation of Monday through Thursday dosing of 0.1 mg/kg (solid line = simulated mean sirolimus concentration; shaded area bound by dotted lines = simulated range of sirolimus concentration). The simulated sirolimus AUC data was generated from the walk-in study of sirolimus. Figure 3b. displays sirolimus concentrations in whole blood shown as the average value and 95% CI, obtained from dogs receiving 0.1 mg/kg of sirolimus orally within the SOC + S clinical trial, at the 2- and 8-hour time points across the 4 cycles of drug treatment (C1 D11 = Cycle 1, Day 11; C1 D25 = Cycle 1 Day 25; C2 D25 = Cycle 2 Day 25; C3 D25 = Cycle 3 Day 25; C4 D25 = Cycle 4 Day 25). ANOVA analysis shows that the 2 and 8 hour time points for each cycle are significantly different (* denotes significant difference, p < 0.0001) but none of the 2 or 8 hour average values were significantly different (p = 0.4825) when compared across dosing cycles.

Figure 4:

Kaplan-Meier event curves depicting the probability of survival with respect to disease-free interval (a) and overall survival (b) between the SOC (blue line) and SOC + S (red line) arms of the trial.