Ultrasonography of Osteosarcoma in Dogs
Sonography Atlas of Dogs

Abstract

Dosing of toceranib

Dogs were initially assigned a dose of either 2.5 mg/kg or 2.75 mg/kg. However, given tablet size limitations, the dose rates were not exact, and this was further compounded by changes in body weight during the study as well as the need for an occasional dose adjustment due to AEs. Table 2 shows the actual mean doses of toceranib for dogs enrolled in the study determined by averaging the dose calculated over the four study visits. These are grouped by range of dose, and the number of dogs in each dose range is provided. The number of dogs requiring a dose modification was five for those initiated at 2.5 mg/kg and three for those initiated at 2.75 mg/kg.

Table 2: Toceranib dosing groups

Response to therapy

Tumors were assessed at baseline and again on days 7, 14 and 30 or at the time of suspected progression by direct measurement, thoracic radiography, abdominal ultrasound or CT. SD over 30 days was observed in 31/40 dogs (77.5%) and 5/40 dogs experienced PR to therapy (12.5%) for an overall clinical benefit rate of 90%. There were 4 dogs (10%) that developed PD during the course of treatment. These data are summarized in Table 3.

Table 3: Response to therapy by tumor typePD Progressive Disease SD, Stable disease PR Partial response.

Adverse events

Drug-related AEs were limited and in most cases readily treatable with supportive care. Using the VCOG-CTCAE classification [9] there were 188 grade 1, 27 grade 2, 8 grade 3, and 1 grade 4 AEs (Table 4); these are described below:

Table 4: Adverse events in toceranib treated dogs

Gastrointestinal

Fifteen dogs (52.5%) developed grade 1 diarrhea during the study, 10 of which experienced more than one episode. Six dogs developed grade 2 diarrhea, and two had grade 2 colitis. Eighteen dogs (37.5%) developed grade 1 vomiting, 5 of which experienced more than one event; only 3 dogs experienced grade 2 vomiting. Thirteen dogs (32.5%) developed grade 1 anorexia and 1 had grade 2 anorexia. Lethargy was encountered in 13 dogs (32.5%), 11 of which were grade 1 episodes with the remainder grade 2 in severity. Only 3 dogs had weight loss, ranging from 5-10% of their initial recorded body weight, consistent with a grade 1 event.

Analysis of toceranib plasma concentrations

A primary objective of this study was to evaluate the presumed Cmax of toceranib given at doses below that indicated on the label (3.25 mg/kg) to determine whether these doses were associated with concentrations of drug likely to result in biologic activity. In prior studies the Cmax was determined to be approximately 6–8 hours post drug administration [7,8]. Additionally, a pharmacodynamic study in dogs demonstrated that plasma concentrations of toceranib 8 hours post drug administration ranging from 30 to 180 ng/ml were associated with target modulation in vivo[5].

The plasma concentrations of drug at 6 and 8 hours were measured and the highest value was used as the presumed Cmax at days 0, 7 and 14. The plasma samples taken at day 30 over 12 hours confirmed that this was true for 69% (28) of the dogs who had a Cmax at 6 or 8 hours. Of the 12 dogs with Cmax at 12 hours, the reported Cmax ranged from 51% to 98% of the 12 hour concentration: 5 were at > 90%, 8 were > 80%, 10 were > 75% of the 12 hour concentration. Figure 1 shows representative plasma toceranib concentrations at day 30 from 4 dogs. Therefore, for all dogs the plasma concentration reported as Cmax was the true maximum plasma concentration of toceranib post administration, or a value < 25% lower than the Cmax. These values are shown in Figure 2, grouped by dose range. The presumed Cmax plasma concentration of toceranib increased gradually in most dogs over the first 2 weeks of dosing despite the EOD regimen and appeared to level out by day 30. Toceranib dose ranges of 2.4-2.9 mg/kg were associated with mean presumed Cmax values of 100–200 ng/ml by day 30, well above the 8 hour 40 ng/ml plasma concentration considered to be sufficient for target inhibition in vivo. Lastly, there did not appear to be significant differences in drug exposure in dogs dosed between 2.4-2.9 ng/ml by day 30 of dosing.

Figure 1: Toceranib plasma concentrations on day 30 of treatment. Shown are representative plasma toceranib concentrations from 4 dogs obtained on day 30 over 12 hours following drug administration. In this study, 28/40 dogs had Cmax at 6 or 8 hours on day 30. Two of the dogs shown experienced Cmax at 8 hours while the other two experienced Cmax after this time.

Figure 2: Toceranib Cmax concentrations. Shown are the median Cmax concentrations of dogs treated with toceranib in this study. These are presented for each time point of collection (days 0, 7, 14, and 30) by dose range. For all dose ranges above 2.4 mg/kg, Cmax was above the 40 ng/ml concentration of toceranib predicted to be associated with effective target inhibition.

Plasma VEGF

Plasma was collected from dogs 8 hours post dosing on days 0, 7, 14, and 30 and analyzed by ELISA for VEGF concentrations. As shown in Figure 3, although plasma VEGF baseline concentrations were generally very low in most dogs (10–40 pg/ml) they did increase significantly over the study period (p < 0.001). Furthermore, plasma VEGF concentrations appeared to plateau over days 14 to 30, potentially correlating with the leveling out of toceranib drug peak plasma concentrations. Change in VEGF was associated with Cmax (p < 0.001), but not Tmax (p = 0.74).

Figure 3: Change in plasma VEGF in toceranib treated dogs. Plasma VEGF concentrations were assessed in dogs prior to toceranib therapy then at 8 hours post dosing on days 1, 7, 14, and 30 of the study. The percent change in plasma VEGF over the 30 day study period was then assessed. Plasma VEGF concentrations increased significantly from baseline over the 30 day study period (p < 0.001). Furthermore, plasma VEGF concentrations appeared to plateau over days 14 to 30. Change in VEGF was associated with Cmax (p < 0.001) but not Tmax (p = 0.74).

Discussion

Toceranib (Palladia), a multitargeted receptor tyrosine kinase inhibitor was approved by the FDA in 2009 for the treatment of canine MCTs. Based on the pivotal clinical field study the subsequent product label indicated that drug should be administered at 3.25 mg/kg EOD with guidelines for dose reduction as needed to address AEs [1]. However, the initial Phase I clinical trial of toceranib suggested that a dose of 2.5 mg/kg was associated with equivalent biologic activity as that achieved with the MTD [3]. A more recent retrospective study demonstrated significant clinical benefit when toceranib was administered at a median of 2.8 mg/kg [6].

In the current study, dogs with solid tumors were administerd toceranib at an intended dose of 2.5 or 2.75 mg/kg EOD and AEs, response to therapy and plasma concentrations of toceranib and VEGF were assessed. Pharmacokinetic analysis showed that for dogs dosed in the 2.4-2.9 mg/kg range, there was no significant difference among dosing groups with respect to the 6 or 8 hour drug concentrations (presumed Cmax) at each time point tested. Dogs dosed at 2.30-2.39 mg/kg appeared to have lower concentrations of drug at all time points, although there were only 3 dogs in this dose range. Median plasma concentrations of toceranib measured at 6 or 8 hours increased over the first two weeks of therapy for dogs dosed between 2.4-2.9 mg/kg, before leveling off at 100–120 ng/ml. These results differ from those obtained in healthy beagle dogs, where toceranib Cmax was proportional to doses ranging from 2.0 to 6.0 mg/kg [7]. The reason for this is not clear, however it is possible that some differences in drug absorption may occur among dogs with a variety of co-morbidities (such as cancer) compared to that found in laboratory healthy research dogs. Given the studies in mouse models and dogs with MCT demonstrating target inhibition at toceranib plasma concentrations at or above 40 ng/ml [2,4] dosing of toceranib at 2.4-2.9 mg/kg results in plasma concentrations of drug likely to effectively inhibit target receptor tyrosine kinases in vivo.

A substantial motivation for assessing whether lower doses of toceranib can be used in dogs with cancer arises from the observed AE profile of the drug. Known AEs include those involving the gastrointestinal tract (anorexia, diarrhea, vomiting), hematologic system (neutropenia, anemia) and neuromuscular system (lameness, weakness), with hepatopathy, proteinuria, hypertension and pancreatitis less frequently reported [1,3]. In the retrospective study of toceranib in solid tumors, while the AEs reported were similar to those established in the pivotal field study, the rates of anorexia, vomiting, lethargy and neutropenia were substantially lower [6].

While the ability to compare toxicities among the published clinical trials evaluating toceranib in dogs with cancer is limited due to variability in dose rates and dosing schedules used, it does appear that the use of lower doses of drug are associated with significantly fewer grade 3 and 4 AEs. Similar to prior studies, the main AEs observed were GI, hematologic (neutropenia) and musculoskeletal in nature and the majority (86%) were either grade 1 or grade 2 in severity (74% and 12%, respectively). However, the number of grade 3 and 4 adverse events was very low, and 3 of these 9 events (pancreatitis, hypertension and ileus) occurred in the dog that experienced an episode initially characterized as pancreatitis that was not confirmed on histopathology following necropsy. Three additional dogs experienced grade 3 and 4 AEs and only the grade 3 lameness was associated with clinical effects. Importantly, no grade 3 or 4 GI adverse events were documented; this is in contrast to the pivotal field study using 3.25 mg/kg dosing in which grade 3 or 4 diarrhea, vomiting and anorexia occurred in 6.9%, 9.2% and 6.2% of treated dogs, respectively [1].

In the current study we documented AEs not previously formally noted with toceranib administration, although these have been reported anecdotally including PLN, hypertension and a presumed hand-foot like syndrome. Hypertension and PLN are AEs known to occur with the administration of anti-angiogenic agents to human cancer patients. In human patients with renal cell carcinoma (RCC) receiving the closely related tyrosine kinase inhibitor sunitinib, 24% developed hypertension and 8% of these were classified as grade 3 in nature [11]. In the current study, only 3 dogs developed hypertension, 2 of which were classified as grade 3. This clinical toxicity may have been underreported as blood pressure was measured only in animals showing signs consistent with systemic hypertension or illness. While the exact mechanism of hypertension associated with toceranib is not clear, it may be directly related to inhibition of VEGF signaling pathways [12]. Additionally, anti-angiogenic drugs used in human patients have been shown to induce increased concentrations of endothelin-1, and this may contribute to the observed hypertension [13-15].

The development of PLN in dogs treated with toceranib has not been described in previous clinical trials, although in these studies routine urinalysis was not performed at study visits. In the current study, urinalysis was similarly only performed at baseline and not repeated unless clinical signs dictated the need to do so. As such, the actual incidence of PLN may have been underreported. In human cancer patients that receive sunitinb the incidence of PLN has been reported to be approximately 2.5% [16-19]. The mechanisms through which VEGF/VEGFR signaling inhibitors cause proteinuria are not well understood, but several have been proposed including the loss of healthy, fenestrated glomerular capillaries which seems to be a direct consequence of blocking VEGFR signaling and possibly disruption of podocyte integrity [16-19].

In this clinical trial, only 8 dogs (20%) had their dose changed throughout the study period (drug holiday and/or dose reduction) compared to 48.3% in the pivotal field study which used the label dose of 3.25 mg/kg [1]. Although limitations exist when comparing data from different studies, it does appear that the reduced AE profile associated with lower doses of toceranib results in fewer changes to the treatment regimen than those that occur using the MTD.

While tumor response was not one of the main objectives of this study, clinical benefit was observed in 36/40 (90%) of the patients, with 12.5% experiencing PR and 77.5% experiencing SD; only 10% of dogs developed PD. Furthermore, 35 of the 40 dogs remained on toceranib for an average of 16.7 weeks after day 30. During this time, one dog with a thyroid carcinoma transitioned from SD to a PR and 3 dogs transitioned to CR (two SCC and one salivary gland adenocarcinoma). These data support the notion that administration of toceranib at doses ranging from 2.4-2.9 mg/kg is associated with meaningful clinical benefit in dogs with cancer.

In human patients administered sunitinib, concentrations of circulating endothelial precursors and plasma VEGF have been shown to increase over time during treatment [20]. Unlike toceranib, sunitinib is generally administered daily for 28 days, followed by a rest period of 14 days, and these concentrations return to baseline during the rest period. It has been hypothesized that continual inhibition of the VEGF/VEGFR signaling axis results in a feedback mechanism promoting an enhanced angiogenic response [21]. The increase in VEGF during treatment with sunitinib or other VEGF/VEGFR inhibitors is generally believed to be a surrogate biomarker of pathway inhibition. In the current study we found that plasma VEGF concentrations increased over time despite using toceranib at the lower dose and giving drug on an EOD basis. This finding further supports the premise that lower doses of toceranib are associated with biologically relevant plasma concentrations of drug.

Conclusions

In summary, toceranib administration at doses between 2.4-2.9 mg/kg EOD results in plasma drug concentrations associated with expected receptor kinase inhibition. This is supported by the fact that 90% of treated dogs experienced clinical benefit over the 30 day treatment period and 35/40 dogs continued on toceranib following study conclusion. Importantly, AEs associated with toceranib administration at the lower doses were well tolerated with no grade 3 or 4 GI events, compared to those reported with dosing at the MTD of 3.25 mg/kg. Together, these data support the use of toceranib at doses ranging from 2.4-2.9 mg/kg.

Abbreviations

VEGF: Vascular endothelial growth factor; VEGFR2: Vascular endothelial growth factor receptor-2/KDR/Flk-2; PDGFRα: Platelet derived growth factor receptor-alpha; PDGFRβ: Platelet derived growth factor receptor-beta; KIT: Stem cell factor receptor; RTK: Receptor tyrosine kinase; EOD: Every other day; AGASACA: Apocrine gland anal sac adenocarcinoma; CR: Complete response; PR: Partial response; PD: Progressive disease; SD: Stable disease; Cmax: Maximum plasma concentration of the drug; Tmax: The time after administration of a drug when the maximum plasma concentration is reached; AUC: Area under the curve.

Competing interests

CL has received honoraria from Pfizer Animal Health (Zoetis) for continuing education events related to toceranib, and for consulting activities related to the use of toceranib in dogs with cancer.

Authors’ contributions

LFB assisted with patient accrual and management, performed several of the required analyses and wrote the draft of the manuscript. RP, SB, and WC assisted with patient accrual and management. MP performed the statistical analyses for VEGF and PK associations. MY performed the toceranib PK determinations. CAL conceived the clinical trial, oversaw all aspects of the study, and completed the manuscript. All authors read and approved the final manuscript.

Acknowledgments

This clinical trial was supported by an unrestricted grant from Pfizer Animal Health (now Zoetis). The Clinical Trials Office (CTO) at the Ohio State University College of Veterinary Medicine coordinated all aspects of this study including generation of case report forms, collation of data and quality assurance. This project was supported by Award Number Grant 8UL1TR000090-05 to Ohio State University from the National Center for Advancing Translational Sciences. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Center for Advancing Translational Sciences or the National Institutes of Health.

References

Share this article / Teilen Sie diesen Artikel