Evaluation of adverse events in small‐breed dogs treated with maropitant and a single dose of doxorubicin

Abstract Background The recommended doxorubicin (DOX) dose for small dogs is 1 mg/kg. Recent data suggest that DOX‐induced gastrointestinal (GI) toxicosis can be reduced with maropitant treatment. Objectives To investigate the incidence of adverse events (AEs) in small‐breed dogs administered a single 25 mg/m2 DOX followed by administration of maropitant (DOX25). The primary aim was to assess myelo‐ and GI toxicoses for 2 weeks after DOX administration. The secondary aim was to compare the incidence and grades of AEs found in the DOX25 group with a historical control group (DOX 1 mg/kg without administration of antiemetic or antidiarrheal medications). Animals Nineteen small‐breed tumor‐bearing dogs. Methods A prospective, observational study of tumor‐bearing dogs, weighing 5 to 10 kg, administered a single 25 mg/m2 dose of DOX IV, followed by administration of maropitant for the next 5 days. Results Inappetence, vomiting, and diarrhea were found in 7/19, 2/19, and 6/19 of the DOX25 dogs, respectively. Neutropenia and thrombocytopenia was 12/19 and 3/19, respectively. Most AEs were grades 1 and 2, except for grades 3 and 4 inappetence and neutropenia in 3 and 4 dogs, respectively. Furthermore, febrile neutropenia occurred in 3/19 dogs in the DOX25 group. All AEs between the DOX25 and historical control groups were not significantly different. Conclusions and Clinical Importance Vomiting and diarrhea were deemed acceptable with 25 mg/m2 DOX followed by maropitant treatment in 5 to 10 kg dogs; however, additional supportive care might be needed for dogs with inappetence and neutropenia.


| Study animals
This prospective observational study included dogs weighing 5.0 to 10.0 kg that were presented to the Japan Small Animal Cancer Center at the Japan Small Animal Medical Center between May 2012 and September 2017. Dogs were included in the study if they were to receive an initial DOX dose of either a single agent or as part of a multiagent protocol for the treatment of various malignancies diagnosed by cytology, histopathology, or diagnostic imaging (ie, cardiac hemangiosarcoma). Dogs were excluded from the study if they had an earlier history of DOX treatment, received any antiemetic or antidiarrheal agents after DOX treatment, or had preexisting GI toxicosis before DOX administration, such as inappetence, vomiting, and diarrhea, or had a life expectancy of less than 1 month. However, we did not restrict drugs that were related to an underlying disease and had been prescribed before DOX administration or drugs prescribed for febrile neutropenia (FN) prophylaxis. Complete blood count, serum chemistry panels, thoracic radiographs, and abdominal ultrasound imaging were performed in all dogs at the initial visit for staging. Before the study, written informed consent was obtained from all owners.

| Study design
All dogs were administered a 25 mg/m 2 DOX dose (Doxorubicin Hydrochloride; ADRIACIN Injection 10, Aspen Japan, Tokyo, Japan) by IV infusion over 30 to 60 minutes without any premedication. Doxorubicin was diluted in 25 to 100 mL physiologic saline. Maropitant (Maropitant citrate monohydrate, Cerenia, Zoetis Japan, Inc, Tokyo, Japan) was administered PO once a day at a dose of 2 mg/kg for 5 days from the day after DOX administration. To assess the general condition of the dogs, owners were asked to record their dog's activity level, appetite, and water intake throughout the day on a 10-point scale. We asked the owners to measure body temperatures by rectal temperature, heart rates by palpable heartbeat in the left thorax or femoral artery, and respiratory rates by abdominal movement with breathing.
All measurements were taken at rest and at home twice a day. The measured items were written on a sheet and submitted weekly. For GI toxicosis evaluations, we asked about the severity of vomiting (frequency and duration per day) and diarrhea (a 4-level severity scale about the frequency and duration per day) for 2 weeks after DOX administration. To assess myelotoxicosis, CBC was performed 7 and 14 days after DOX administration to determine the incidence of neutropenia and thrombocytopenia. Febrile neutropenia was defined as a body temperature of ≥39.2 C and neutrophil count of ≤2500 cells/ μL, 15 and its incidence during the study period was recorded. Gastrointestinal toxicosis and myelotoxicosis were graded according to the Veterinary Cooperative Oncology Group Common Terminology Criteria for Adverse Events (VCOG-CTCAE v1.1). 16 Treatments for AEs were performed at the discretion of attending veterinarians and was not standardized. In addition, the incidences of GI toxicosis and myelotoxicosis were compared with those in the historical control group.

| Historical control group inclusion criteria
The historical control group consisted of dogs weighing 5 to 10 kg that were presented to the Japan Small Animal Cancer Center at the Japan Small Animal Medical Center, or Kobayashi Animal Hospital, Saitama, between January 2003 and December 2009. These dogs received DOX at 1 mg/kg to treat various malignancies and received no antiemetics or antidiarrheals after DOX administration. However, we did not restrict drugs related to an underlying disease and had been prescribed before DOX administration, or drugs prescribed for FN prophylaxis. We obtained signalment, tumor types, concomitant medications, presence of vomiting or diarrhea, and CBC on days 7 and 14 after the DOX administration.

| Statistical analysis
For statistical comparison between the DOX25 and historical control groups, the Wilcoxon rank-sum test was used for age and body weight. The Chi-square test or Fisher's exact test was used for sex ratios and AE incidences. Differences were considered significant at a P < .05. All statistical analyses were performed using Stata Statistical Software Ver. 14.2 (StataCorp, College Station, Texas).

| Study demographics
A total of 19 dogs were included in the DOX25 group. The median age was 10 years (range, 5-14 years). There were 3 intact and 6 castrated males and 2 intact and 8 spayed females. There were 9 Miniature Dachshunds, 2 Miniature Schnauzers, and 1 of a different breed.
The mean body weight was 6.5 kg (range, 5.03-10.0 kg). The most common tumor type was high-grade lymphoma (multicentric = 4, hepato-splenic = 2), followed by hemangiosarcoma (right atrium = 1, subcutaneous = 2, spleen = 1), transitional cell carcinoma of the bladder (n = 3), subcutaneous soft tissue sarcoma (n = 2), sarcoma (spleen = 1, liver = 1), mandibular osteosarcoma (n = 1), and mammary gland adenocarcinoma (n = 1). As for the disease status at the time of DOX administration, 4 dogs with multicentric lymphosarcoma (LSA) and 1 dog with hepato-splenic LSA had complete remission, and 1 dog with hepato-splenic lymphoma did not have abdominal ultrasonography or a cytologic examination immediately before the DOX administration because of a complete resolution of clinical signs. For tumors other than lymphoma, 4 dogs had gross lesions, and 9 had microscopic lesions at the time of DOX administration. Six dogs received DOX as part of a multidrug protocol, and 13 dogs received DOX as a single agent. The mean and median doses in the DOX25 group were 8.9 and 8.8 mg, respectively. If these dogs had been administered 1 mg/kg dose, the mean and median dose would have been 6.7 and 6.5 mg, respectively. Compared with DOX at 1 mg/kg, the mean dose intensity of DOX at 25 mg/m 2 was 1.32 times higher and the median was 1.35 times higher. Complete blood count on day 7 were performed on day 7 after the DOX administration in 14 dogs and on day 7 ± 1 in the remaining 5 dogs for owner convenience.
Complete blood count on day 14 were performed on day 14 in 11 dogs and on day 14 ± 2 in the remaining 8 dogs for the same reason.

| Adverse events
The incidence of signs of GI toxicosis in the DOX25 group was 7/19 for inappetence, 2/19 for vomiting, and 6/19 for diarrhea.
Most GI toxicosis assessments were grades 1 and 2, except for grade 3 inappetence in 3 dogs ( Table 1). The incidence of neutropenia and thrombocytopenia in the DOX25 group was 12/19 and 3/19, respectively. The majority of neutropenic events were grades 1 and 2, except for grades 3 and 4 neutropenia observed in 1 and 3 dogs, respectively ( One dog in the historical control group had a gastrostomy tube and was excluded from the analysis of inappetence.

| Demographics of the historical control group
A total of 18 dogs were included in the historical control group. The

| Comparison of AEs with the historical control group
The incidence of GI toxicosis in the historical control group was as follows: 7/17 for inappetence, 5/18 for vomiting, and 9/18 for diarrhea.
There was no significant difference compared with the DOX25 group for any GI toxicosis ( Table 1). The incidence of neutropenia and thrombocytopenia was 6/18 and 1/18, respectively, and were grades 1 and 2 in all cases. There was no significant difference compared with the DOX25 group for myelotoxicosis ( Table 2). The incidence of FN in the historical control group was 1/18, and the dog had a grade 2 neutropenia. There was no significant difference in the incidence of FN between the 2 groups (P = .60; Table 2).

| DISCUSSION
Our results show that all vomiting and diarrhea was grades 1 and 2 range and was clinically acceptable when DOX 25 mg/m 2 was administered in combination with maropitant to dogs weighing 5 to 10 kg. On the other hand, inappetence was seen in 36.8% of the dogs, 3 of which were grade 3. In the historical control group, inappetence was also seen in 41.2% of the dogs, 3 of which were grade 3. These results suggest that inappetence after DOX administration should be a concern at any dose. Regarding neutropenia in the DOX25 group, 3 dogs had grade 4 neutropenia, which developed into FN in all of these dogs. This suggests that close monitoring, including temperature pulse respiration measurements at home and supportive care, such as prescription of prophylactic antibiotics, are needed when DOX 25 is administered to dogs weighing 5 to 10 kg.
We showed that vomiting in small dogs weighing 5 to 10 kg, treated with DOX at 25 mg/m 2 with concomitant maropitant, was acceptable. The incidence of vomiting is proportional to dose intensity, 8 meaning that higher dose intensities are associated with greater incidence of vomiting. Therefore, because the incidence of vomiting, which was expected to be higher in the DOX25 group, was actually comparable with that in the DOX 1 mg/kg (historical control) group, it might suggest that maropitant administration with DOX could reduce vomiting at this higher DOX dose in dogs weighing 5 to 10 kg. In the present study, the incidence of vomiting was 10.5% (2/19), and all grades of vomiting were mild. In addition, we did not find a significant difference in the incidence of vomiting between the DOX25 and historical control groups DOX 1 mg/kg without antiemetics or antidiarrheals. However, the results should be interpreted with caution, as the incidence in the DOX25 group might not have been completely comparable with that of the historical control group as a type II error because of small sample sizes. Also, the lack of a significant difference does not mean that they are equivalent.
Our results suggest that maropitant could also reduce diarrhea caused by DOX. The concomitant use of maropitant with DOX could also reduce diarrhea and vomiting. 9 Neurokinin 1 receptors, where maropitant binds, exist not only in the central chemoreceptor trigger zone but also in the intestinal tract. 17,18 Other studies have shown that substance P plays a significant role in the development of diarrhea in ulcerative colitis and Crohn's disease by binding to NK1 receptors. 19,20 These facts suggest that NK1 receptor antagonists, such as maropitant, can also act on peripheral receptors to inhibit the development of diarrhea. The incidence of diarrhea in the present study was comparable to that reported previously (33%) 9 but showed no significant difference between the DOX25 and historical control groups. In any case, all diarrheal events observed in the 6 dogs of the present study were grades 1 and 2 and considered clinically manageable.
Our data showed that maropitant effectively reduced the incidence of DOX-induced vomiting but did not improve DOX-induced inappetence.  and Dr. Wendy Sprague for English language assistance.