Dogs enrolled in this trial were generally representative of the overall population of client-owned dogs with advanced-stage macroscopic MCT in terms of age, breed, gender, tumor presentation and grade. No novel adverse events were noted in dogs receiving the two-drug combination that would not have been predicted based on the known event profiles for each drug used as a single-agent. However, the intensity of myelosuppression, specifically neutropenia, was found to be unexpectedly high following simultaneous combination when compared to each drugs known adverse event profile when used alone. This resulted in our determination that the maximally tolerated dose and schedule for the combination is vinblastine administered at 1.6 mg/m2 every other week with toceranib used at the FDA label dose (3.25 mg/kg PO, every other day).
Two dogs in each of the first two cohorts experienced dose-limiting grade 4 neutropenia and all dogs in the remaining cohorts experienced grade 1 – 3 neutropenia at day 7 that necessitated extension of vinblastine dosing intervals to every other week. As neutropenia represents the established DLT for vinblastine in dogs and the starting cohort dose of toceranib was already at the low end of that documented to result in anti-MCT activity in phase I/II trials when used as a single agent (17
), a decision was made to decrease vinblastine in subsequent cohorts rather than decrease the toceranib dose. This strategy was later confirmed as increasing the dose of toceranib did not exacerbate neutropenia in cohort 4 and 5 when the vinblastine dose was held constant.
The 1.6 mg/m2
every other week dosing regimen established as maximal for vinblastine in this combination equates to a dose intensity (DI), defined as mg/m2
/week regardless of schedule, of 0.8 mg/m2
/wk. This represents a 54 – 71% reduction in DI when compared to that of published vinblastine monotherapy (DI of 1.75 – 2.6 mg/m2
). This degree of DI reduction required for the simultaneous combination is contrary to the current paradigm of only combining anticancer drugs if their simultaneous use allows maintaining DI known to be effective in the single-agent setting.
It is likely that toceranib sensitizes the myeloid compartment to the effects of vinblastine by a process that is not yet understood. This is not a novel phenomenon and has been seen when tyrosine kinase inhibitor therapy and cytotoxic chemotherapy have been combined in human clinical trials (30
). In people, similar findings were observed when combining gefitinib and vinorelbine or gemcitabine. High rates of grade 3–4 adverse events, including 72% of people developing neutropenia, were observed on this combination, leading to early closure of the study (30
). Similar observations were made when erlotinib and vinorelbine were combined in a phase I study which resulted in 50% grade 3–4 neutropenia and 25% febrile neutropenia that precluded adequate delivery of therapy (31
). Further, studies combining vinca alkaloids with EGFR inhibitors demonstrated unacceptable myelosuppression (32
). It has been suggested that one potential mechanism for this hematologic toxicity may be that both agents are metabolized by CYP3A4 enzymes, resulting in prolonged exposure to the active drugs (33
). The cytochrome P450 hepatic enzyme system is thought to play a major role in toceranib metabolism (35
) and the label insert advises against co-administration of toceranib with strong inhibitors of CYP3A4 enzyme systems. While this is plausible, pharmacokinetic modeling of the combination would be necessary to substantiate this interaction. As toceranib affects several cellular signal transduction pathways involved in cell cycling, proliferation and apoptosis, other mechanisms of synergistic myelosuppression could also be involved. For example, this class of drug does inhibit colony stimulating factor-1 receptor, and while toceranib did not result in clinically significant neutropenia (i.e., grade 3 or 4) and was not dose-limiting in the field trial (20
), this effect may exacerbate the level of neutropenia when combined with cytotoxic drugs. Importantly, several TKIs have been shown to inhibit ATP-binding cassette (ABC) multidrug transporters (MDR-ABC proteins) and the pattern of MDR-ABC transporter-TKI interactions may also help to understand the general pharmacokinetics and toxicities of new TKIs used in combination with standard cytotoxic drugs (36
). In particular, sunitinib, a TKI that was co-developed along with toceranib which targets identical TKI receptors has been shown to inhibit these multidrug transporter systems (37
). Finally, a similar c-kit inhibiting TKI, imatinib mesylate has been shown to interact with MDR-ABC proteins in hematopoietic stem cells (39
). Taken in totality, such TKI-MDR interactions may explain the exacerbation of myelosuppression noted with the combination under study here and warrant further interrogation.
While the extent and durability of antitumor activity is not a primary objective of phase I dose-finding trials of this design, the combination of vinblastine /toceranib produced objective clinical response in 71.4% of dogs and some were durable. Responses were observed in all dosing cohorts evaluated. This response rate is higher than that reported in two studies using single agent vinblastine at 2 mg/m2
(12% objective response rate) (16
) and also higher than that reported in the phase II field trial of single agent toceranib (20
). While this response rate appears to compare favorably to that reported for single-agent vinblastine or toceranib, the degree to which activity relates to each agent alone, or the potential synergistic anti-tumor activity of the combination would require a prospective randomized clinical trial comparing single and combination vinblastine and toceranib therapies. Based on the significant response rate and enhanced myelosupression observed with the combination, one could theorize that the combination, even with the requirement for decreased DI of vinblastine, may result in additive or synergistic antitumor response that may have clinical utility. The previously discussed interaction between TKI agents and MDR-ABC proteins that may be at play in the enhanced myelosuppression observed with the combination may also theoretically result in synergistic interactions between toceranib and MDR-substrate cytotoxics and warrants further in vitro
and in vivo
interrogation. A prospective randomized clinical trial, comparing single and combination treatment arms would be required to confirm or refute any additive or synergistic antitumor activity from the combination. Until such a study is performed, there is insufficient information to recommend the combination over sequential use of vinblastine followed by maintenance toceranib, which is currently employed by many veterinary oncologists for similar staged macroscopic MCT in dogs.
Limitations of the current study are similar to those inherent in phase I dose-finding trials in general. As numbers are small and dose-intensity of cohorts vary, a more accurate assessment of response rates, response durability, and long-term adverse event profiles would await larger phase II or III assessments. That being said, observed antitumor activity is likely somewhat more accurate in this study as only de-escalations of vinblastine occurred rather than dose-escalations as is more common in phase I trials when below-MTD cohorts are prevalent early in trial. The lack of consistent clinical staging was an additional minor limitation, as this would likely only lead to under-staging and since adverse event profiles are known to be generally higher in more advanced disease, this would not have resulted in an underestimation of events. Finally, no attempt was made to interrogate the Kit mutation status of tumors in this trial. While this certainly may affect response rates for MCT in general, activity is not the primary endpoint for this study design.