Results of this study were presented in part at the Veterinary Cancer Society 28th Annual Conference, Seattle, WA, 2008
Corresponding author: N.C. Northrup, DVM, DACVIM (Oncology), Department of Small Animal Medicine and Surgery, University of Georgia, College of Veterinary Medicine, 501 D.W. Brooks Drive, Athens, GA 30602; e-mail: email@example.com.
Administration of streptozotocin (STZ) at a 21-day interval has been described in dogs with stage II and III insulinoma. Myelosuppression was not observed, suggesting the possibility of increasing dose intensity by decreasing the interval between doses.
To describe the tolerability of a biweekly STZ protocol. A secondary objective was to describe the outcome of dogs treated with this protocol.
Nineteen dogs with residual local, metastatic, or recurrent insulinoma.
After surgery for insulinoma, or at the time of recurrence, dogs were treated with a previously described STZ and saline diuresis protocol. Treatments were administered every 14 days. All dogs received antiemetic treatment. Adverse events (AEs) were recorded and graded. Outcome endpoints assessed were progression-free survival (PFS) and survival.
None of the dogs experienced neutropenia or thrombocytopenia. Mild to moderate gastrointestinal toxicity was the most common AE. Diabetes mellitus was observed in 8 dogs and, in 6, resulted in euthanasia or death. Two dogs developed nephrotoxicity manifested as Fanconi syndrome in 1 and nephrogenic diabetes insipidus in the other. Six dogs developed increased alanine amino transferase activity. Hypoglycemia at the end of the STZ infusion, resulted in collapse in 1 dog and a generalized seizure in another. The median overall PFS and survival time were 196 and 308 days, respectively.
Conclusions and Clinical Importance
Streptozotocin can be safely administered to dogs with insulinoma, but serious AEs are possible. Additional investigation is required to better define the role of STZ in managing dogs with insulinoma.
Insulin-secreting tumors of pancreatic β cells in dogs are most commonly malignant and metastasize to regional lymph nodes and the liver.[1-4] Reported survival times (STs) are highly variable. Although resolution of hypoglycemia and long-term survival are possible after surgical excision,[4-8]the majority of dogs are euthanized because of recurrent hypoglycemia and associated weakness, ataxia, peripheral neuropathy, seizures, and collapse.[1-9] Dogs that remain hypoglycemic postoperatively have shorter STs than those that are hyperglycemic or normoglycemic,[1, 3, 7] and more advanced clinical stage has been associated with shorter time to recurrence of hypoglycemia and ST. An effective antineoplastic therapy is needed for dogs with incompletely resected or metastatic insulinoma.
Streptozotocin1 (STZ) is an alkylating agent in the nitrosourea family. It is similar enough in structure to glucose to be taken up by the glucose transporter 2 (GLUT2) transmembrane carrier protein, but not by other glucose transporters. Because pancreatic β cells have high concentrations of GLUT2 transporters, STZ is selectively toxic to these cells. Streptozotocin has been safely administered every 3 weeks to dogs at a dosage of 500 mg/m2 IV in conjunction with saline diuresis.[11, 12] Dogs with stage II and III insulinoma treated with STZ at 3-week intervals remained normoglycemic for a median of 163 days (95% CI, 16–309 days). Although this was not different from a historical control group treated surgically and symptomatically, ultrasonographically measurable partial remissions were described in 2 dogs with suspected metastasis and 2 dogs experienced resolution of insulinoma-associated polyneuropathy. Observed toxicities of STZ included emesis, increased serum alanine aminotransferase (ALT) activity, increased serum creatinine concentration, and diabetes mellitus (DM). Because hematologic toxicity was not observed, we hypothesized that STZ could be safely administered at 2-week intervals, allowing increased dose intensity and potentially improving disease control and ST. The objective of this study was to evaluate the tolerability of streptozotocin administered every 14 days to dogs with insulinoma. A secondary objective was to describe the progression-free survival (PFS) and survival of dogs treated with this protocol.
Case Selection and Initial Diagnostic Testing
From 2001 to 2008, cases were prospectively treated and monitored at 4 institutions. Dogs provisionally diagnosed with insulinoma based on hypoglycemia with inappropriately high serum insulin concentrations underwent excision of the primary tumor and resectable metastases. Preoperative diagnostic tests included CBC, serum biochemistry profile, urinalysis, serum insulin and glucose concentrations, 3-view thoracic radiography, and abdominal ultrasonography. Diagnosis of insulinoma was confirmed histologically in all dogs.
Criteria for recommendation of adjunctive treatment with STZ included incompletely excised or metastatic insulinoma, persistent hypoglycemia with or without hyperinsulinemia (presumed due to micrometastatic disease), or recurrence of hypoglycemia after resection of insulinoma. Before initiation of the STZ protocol, dogs were evaluated by CBC, serum biochemistry profile, urinalysis, serum insulin and glucose concentrations, and abdominal ultrasonography. Metastasis was confirmed by histology or cytology, and stage was classified according to the WHO system. Stage I indicates tumor confined to the pancreas, stage II indicates lymph node metastasis, and stage III indicates distant metastasis.
A planned dosage of 500 mg/m2 of STZ was diluted in 36.6 mL/kg of 0.9% NaCl and administered IV over 2 hours. Treatments were administered every 2 weeks for a total of 5 doses (Table 1). A previously described 7-hour diuresis protocol (18.3 mL/kg/h of 0.9% NaCl given IV for 3 hours before and 2 hours after STZ) and prophylactic antiemetic treatment was administered with each dose. Antiemetic agents used included butorphanol tartrate2 0.4 mg/kg IM 15 minutes before administration of STZ, dpolasetron mesylate3 0.6 mg/kg IV immediately before STZ, and metoclopramide hydrochloride4 0.4 mg/kg SC or PO q8h starting the morning of STZ therapy, or, starting in 2007, maropitant citrate5 1 mg/kg SC 1 hour before STZ. Antiemetic protocols were not standardized because of the time period over which this study occurred and changed as potentially superior agents became available. At the discretion of the attending clinician, prednisone 0.5–1 mg/kg q24h or divided q12h PO was administered to dogs with persistent hypoglycemia.
Table 1. Two-week interval streptozotocin protocol used to treat 19 dogs with insulinoma
CBC, Biochemistry Profile, Urinalysis, Insulin and Blood Glucose Concentrations
CBC, Blood Glucose, Creatinine, Urine Specific Gravity
CBC, Biochemistry Profile, Urinalysis, Insulin and Blood Glucose Concentrations
CBC, Blood Glucose, Creatinine, Urine Specific Gravity
CBC, Biochemistry Profile, Urinalysis, Insulin and Blood Glucose Concentrations
CBC, Biochemistry Profile
CBC, Biochemistry Profile, Urinalysis, Insulin and Blood Glucose Concentration
Assessment of Toxicity and Outcome
Patients were monitored by physical examinations and diagnostic testing according to the protocol in Table 1. After 17 weeks, follow-up was at the discretion of attending clinicians. Observed toxicity was graded using the Veterinary Co-operative Oncology Group—Common Terminology Criteria for Adverse Events (VCOG-CTCAE) after chemotherapy or biological antineoplastic treatment in dogs and cats, v1.0. PFS was defined as the number of days from the first STZ treatment until recurrence of hypoglycemia, detection of local recurrence or metastasis, or death because of any cause. The presence of metastasis was confirmed cytologically. ST was defined as the time from initiation of STZ therapy until death because of any cause.
The Kaplan–Meier product limit method was used to estimate PFS and ST. Dogs were censored from the PFS analysis if they were alive and had not developed hypoglycemia or detectable tumor recurrence or metastasis by the time of the last follow-up. Dogs that were alive at the last follow-up were censored from the survival analysis. Dogs dying of intercurrent disease were not censored from either analysis because of difficulty determining that death was unrelated to insulinoma and because the most common intercurrent disease causing death was a consequence of therapy DM. Intercurrent disease was defined as any disease occurring during the course of treatment or follow-up of insulinoma. Comparison of the PFS and survival curves of dogs that did or did not develop DM was performed using log-rank tests. Statistical evaluation was performed using SAS V 9.2.6 Tests were 2-sided with significance set at P < .05.
Nineteen dogs were treated according to the protocol in Table 1. Dogs were treated at the University of Georgia College of Veterinary Medicine (n = 12), Cornell University College of Veterinary Medicine (n = 5), Carolina Veterinary Specialists (n = 1), and Veterinary Cancer Group (n = 1). Characteristics of the dogs are summarized in Table 2. Twelve dogs were treated because of the presence of histologically confirmed metastasis at the time of surgery. Five dogs with stage I insulinoma were treated with STZ because of potential increased risk of shorter PFS and survival because of incomplete excision (n = 3), persistent hypoglycemia (n = 1), or persistent hyperinsulinemia (n = 1). Two additional dogs with stage I insulinoma received STZ when hypoglycemia recurred at 115 and 402 days after surgery.
Table 2. Characteristics of 19 dogs with insulinoma treated with streptozotocin at 2-week intervals
Number of Dogs (%)
Some dogs exhibited >1 clinical sign.
Dogs classified as stage III had liver metastases (n = 5) or a mass in the region of the splenic vein confirmed histologically to be insulinoma (n = 1).
The location of residual gross disease was pancreas (n = 2), liver (n = 2), or pancreas and liver (n = 2). Gross disease was observed at surgery and diagnosis confirmed by incisional biopsy and histology.
Jack Russell Terrier
Other purebred dog
Duration of clinical signs before diagnosis (days)
Time between surgery and first streptozotocin treatment (days)
Number of streptozotocin doses received
The planned treatment protocol included 5 doses, but 13 dogs received fewer than 5 doses (Table 2) because of progressive disease (n = 2), adverse events (AEs) (DM [n = 4], increased liver enzyme activity [n = 3], renal injury [n = 2]) or other reasons (owner withdrawal of treatment [n = 1] and death due to splenic hemangiosarcoma [n = 1]). The planned dosage was 500 mg/m2 IV q14d; however, after hospitalization of 2 small breed dogs for grade II emesis, the dosage was empirically decreased for small dogs. Dogs >15 kg received 500 mg/m2 and dogs <15 kg received 375–400 mg/m2. Fifteen dogs initially were treated at 500 mg/m2, 2 at 400 mg/m2, and 2 at 375 mg/m2. Fifteen dogs received concurrent prednisone for hypoglycemia. Five were tapered off prednisone by 13, 18, 27, 161, and 196 days after starting the STZ protocol and the remaining 10 received prednisone for the rest of their lives.
No neutropenia or thrombocytopenia was observed 7 or 14 days after STZ in 18 dogs with available neutrophil counts and 17 dogs with available platelet counts. Although myelosuppression was not observed in any of the dogs, all experienced at least 1 AE. These are summarized in Table 3. Twelve dogs experienced gastrointestinal (GI) signs including nausea, anorexia, vomiting, regurgitation, diarrhea, colitis, or some combination of these (Table 3). GI toxicity occurred immediately or within 4 hours after administration of STZ and resolved in ≤24 hours in all but 2 dogs: 1 with anorexia that lasted for up to 4 days after STZ and another with anorexia and diarrhea 4 days after STZ. Four dogs were hospitalized for management of GI toxicity after the 1st (n = 2 at 500 mg/m2 and n = 1 at 400 mg/m2) or 4th dose (n = 1 at 425 mg/m2) of STZ.
Table 3. Adverse events in 19 dogs with insulinoma treated with streptozotocin at 2-week intervals. Grade is according to the Veterinary Co-operative Oncology Group—Common Terminology Criteria for Adverse Events (VCOG-CTCAE) following chemotherapy or biological antineoplastic therapy in dogs and cats v1.0.
Number of Dogs Affected
For nausea/anorexia, grade 1 indicates loss of appetite without alteration in eating habits or such that coaxing or dietary change needed to maintain appetite and grade 3 indicates loss of appetite of 3–5 days duration; associated with significant weight loss of malnutrition; IV fluids, tube feeding or TPN indicated.
For vomiting/regurgitation, grade 1 indicates <3 episodes in 24 hours, grade 2 indicates 3–5 episodes in 24 hours; <3 episodes/day for >2 days but <5 days; parenteral (IV or SC) fluids indicated <24 hours, and grade 3 indicates >5 episodes in 24 hours; vomiting >4 days; IV fluids or PPN/TPN indicated >24 hours.
For diarrhea/colitis, grade 1 indicates an increase of >2 stools per day over baseline and grade 2 indicates an increase of 2–6 stools per day over baseline; parenteral (IV or SC) fluids indicated <24 hours.
For diabetes mellitus, grade 3 indicates symptoms interfering with activities of daily living; insulin needed and grade 5 indicates resulting in the death of the patient.
For Increased ALT Activity, grade 3 indicates >2.0–10 times the upper limit of normal and grade 4 indicates >10 times the upper limit of normal.
The next most common adverse effect was DM, confirmed in 8 dogs. Hyperglycemia was detected a median of 84 days after initiation of STZ therapy (range, 7–298 days). This was after 1 (n = 1), 3 (n = 1), 4 (n = 3), or 5 (n = 3, 17–62 days after the 5th dose) doses of STZ. Streptozotocin was discontinued when DM was detected if additional doses were planned. Interestingly, 1 dog that remained hypoglycemic and weak after 4 STZ treatments died of diabetic ketoacidosis (DKA) 9 months later, suggesting a delayed response to STZ. Five dogs with DM were treated with insulin. In addition to the dog that died of DKA, 2 dogs were euthanized without treatment 91 and 495 days after STZ. Euthanasia or death was a consequence of DM in 6 dogs at a median ST of 303 days (range, 91–864 days).
Renal injury was observed in 2 dogs. A 10-year old castrated male German Shepherd Dog with nonresectable insulinoma developed polyuria 4 days after the 2nd dose of STZ. Repeatable glucosuria in the absence of hyperglycemia, and hypokalemia were present. Fanconi syndrome was confirmed with a Fanconi syndrome screening panel7 that showed a strongly positive glucose spot test and a strongly positive nitroprusside test. Streptozotocin treatment was discontinued and the dog was euthanized 7 months later for weakness associated with progression of insulinoma. The 2nd dog was a 10-year-old spayed female mixed breed dog. Her urine specific gravity (USG) was 1.033 at the time of the 3rd dose of STZ. At the time of the 4th STZ treatment, her USG was 1.009 with evidence of a urinary tract infection on urinalysis. Serum creatinine and blood glucose concentrations were normal. She was treated and 6 days later was presented for extreme polyuria and polydipsia (PU/PD). Urinary tract infection with Esherichia coli was diagnosed and treated. After 10 days of antibiotic therapy, the urine culture was negative, but PU/PD had progressed. The USG gravity was 1.007 with trace urine glucose despite low serum glucose (60 mg/dL; reference range, 77–120). Renal ultrasonography was normal except for mild bilateral renal pelvis dilatation consistent with polyuria. Acquired nephrogenic diabetes insipidus was suspected based on water consumption of 188 mL/kg/24 h, lack of response to desmopressin acetate, and elimination of other potential causes. Streptozotocin treatment was discontinued. Polyuria and polydipsia with USG between 1.005 and 1.008 and intermittent urinary tract infections continued until euthanasia of the dog 9 months later because of weakness resulting from progression of insulinoma.
Six dogs developed increased serum ALT activity during treatment with STZ (grades presented in Table 3). Increases were noted after 2 (n = 3), 3 (n = 1), 4 (n = 1), and 5 (n = 1) doses. Four of the 6 dogs were receiving prednisone concurrently. Treatment with s-adenosyl methionine8 (20 mg/kg q24h PO) was initiated and STZ discontinued (n = 3) or delayed (n = 1, for 22 days). Increased enzyme activity initially worsened and then improved, but did not normalize. Abdominal ultrasonography was performed in 4 of the dogs with increased ALT activity; the liver appeared normal in 2 dogs and hepatic nodules were identified in 2 dogs. One had hypoechoic nodules cytologically diagnosed as vacuolar change. The other had hypoechoic target-like nodules cytologically diagnosed as metastatic neuroendocrine tumor. Despite the finding of metastasis, increased ALT activity in this dog was attributed to STZ, because ALT activity decreased after discontinuation of the drug. Only 1 dog underwent hepatic biopsy at the time of increased ALT activity. Vacuolar hepatopathy with no evidence of metastasis was diagnosed histologically. This dog underwent necropsy 3 months later after euthanasia for meningioma. Hepatic degeneration and regeneration were identified on histopathology. Bile acid concentrations were not evaluated in any of the dogs.
At the conclusion of this study, 18 dogs were dead and 1 was lost to follow-up at 333 days after initiation of STZ (Fig 1). Seven dogs were euthanized for recurrence or progression of insulinoma. The remaining dogs were euthanized or died as a consequence of DM (n = 6), seizures of undiagnosed etiology (n = 1, blood glucose 308 mg/dL at time of seizure), and other tumors (n = 4; splenic and hepatic sarcoma [with hemoabdomen, presumed to be hemangiosarcoma based on cytology], meningioma [diagnosed by necropsy], mass involving C4 spinal nerve [not biopsied, MRI appearance consistent with nerve sheath tumor], and sarcoma of the trigeminal nerve [presumptive nerve sheath tumor based on MRI appearance and cytology]). Median overall ST was 308 days (range, 20–1404 days; Fig 1) and estimated probabilities of being alive at 1 and 2 years were 41 and 24%, respectively. The median overall PFS was 196 days (range, 20–840 days; Fig 2) with estimated probabilities of being progression-free at 1 and 2 years of 21 and 5%, respectively.
It was not possible to determine response rates to STZ because of the lack of evaluable gross disease in most dogs, confounding normalization of blood glucose concentration because of administration of prednisone, and incomplete follow-up insulin concentration data. At the time of initiation of STZ, residual disease was detectable ultrasonographically in only 2 of the dogs, 1 with hepatic metastases and 1 with a pancreatic mass, enlarged metastatic lymph node, and hepatic metastases. These dogs experienced disease progression 7 and 27 days after STZ, respectively.
Before STZ, 7 of the 19 dogs had normal insulin concentrations, 11 were hyperinsulinemic, and 1 did not have an insulin concentration rechecked. Follow-up insulin concentrations were performed inconsistently at the discretion of attending clinicians. Two of the 11 hyperinsulinemic dogs had normal insulin concentrations 80 and 92 days after initiation of STZ treatment. One developed DM 10 days after the 1st STZ treatment and relapsed with hypoglycemia 286 days later. The 2nd dog relapsed in 1 month.
Overall, hypoglycemia resolved in 15 of 17 dogs treated with surgery and adjunctive STZ and in 1 of 2 dogs treated at the time of recurrence of hypoglycemia. However, 15 dogs received concurrent prednisone. Thus, attribution of resolution of hypoglycemia solely to surgery, STZ, or prednisone was impossible. Four dogs were hypoglycemic at initiation of STZ. Three remained hypoglycemic and the 4th developed DKA 298 days after initiation of STZ. For the 16 dogs experiencing resolution of hypoglycemia, the median PFS was 223 days (range, 54–840 days) and the median survival time (MST) was 360 days (range, 91–1404 days). The 3 dogs that remained hypoglycemic lived (and had a PFS of) 20, 44, and 111 days after initiation of the STZ protocol. The median follow-up time for the 9 dogs that died without recurrence of hypoglycemia was 298 days (range, 91–840 days).
Two dogs were treated with STZ when hypoglycemia and clinical signs recurred at 115 and 402 days after surgery. The first remained hypoglycemic and was euthanized for hemoabdomen secondary to hemangiosarcoma after 2 doses of STZ. The second became normoglycemic, had a PFS of 60 days, and was euthanized for a trigeminal nerve sarcoma 283 days after initiation of STZ.
Seizures resolved in all 10 affected dogs, and episodic weakness resolved in 7 of 9 affected dogs. Eight of 12 dogs with peripheral neuropathy experienced resolution of weakness and gait abnormalities, 3 improved, and 1 remained stable.
Eight dogs developed DM, including 4 of the 7 dogs with normal insulin concentrations at initiation of the STZ protocol. The median PFS of dogs that developed DM was 280 days (range, 91–840 days; Fig 3) and the median PFS of dogs that did not develop DM was 139 days (range, 20–553 days; Fig 3). This difference was not significant. The MST of the dogs that developed DM was 402 days (range, 91–840 days), and the MST of dogs that did not develop DM was 283 days (range, 20–1404 days). This difference was not significant.
Five dogs received treatment at the time of relapse of hypoglycemia. Four were treated with additional STZ and 3 of these dogs later received doxorubicin (n = 1), 5-fluorouracil (n = 1), or both (n = 1). One dog underwent surgical excision of a nodule at the site of the previously resected insulinoma and an associated metastatic lymph node.
The objective of this study was to evaluate a biweekly schedule of administration of STZ for dogs with insulinoma. As expected, myelosuppression was not observed, allowing increased dose intensity. The most commonly observed toxicity was GI, mild to moderate in severity, and abrogated by antiemetic agents. DM was common and resulted in the euthanasia of some dogs because of late diagnosis and owner reluctance or inability to treat. Renal toxicity was uncommon, but severe when it occurred, manifesting as Fanconi syndrome and nephrogenic diabetes insipidus. Increased ALT activity was observed. Activity of STZ against canine insulinoma was supported by resolution of hyperinsulinemia and normalization of blood glucose concentration in some dogs.
The protocol evaluated in this study was based on the protocol described by Moore et al according to which STZ 500 mg/m2 was administered IV with 7 hours of saline diuresis at 3-week intervals to 17 dogs with insulinoma. Our primary objective was to determine the tolerability of biweekly administration of STZ. As expected based on toxicologic evaluation in dogs and the study by Moore et al, neutropenia and thrombocytopenia were not observed in any of the dogs, but AEs occurred in all 19. Similar to what was reported with the every-3-week STZ protocol, the most common toxicity associated with biweekly STZ was GI in nature, typically acute onset (within 24 hours of treatment) inappetence and vomiting or regurgitation. GI toxicity generally was mild to moderate, but 4 dogs required hospitalization for supportive care.
Because, STZ is a diabetogenic agent with selective cytotoxicity for neoplastic and normal pancreatic β cells, DM was an expected toxicity. In the study by Moore et al 12% of the dogs developed DM. By comparison, 42% of the dogs treated biweekly developed DM. Although we cannot draw conclusions when comparing to historical data, this finding suggests more potency with increased dose intensity. We initially considered this to be a positive outcome, because the 2 dogs developing DM in the Moore study were the longest survivors. However, although PFS and ST were longer in dogs that developed DM than in those that did not, the differences were not statistically significant. These analyzes were limited by small sample size, and confounded by DM-related euthanasia or death and the potentially premature cessation of STZ therapy in patients developing DM. It is important to educate owners about DM as a possible adverse effect of STZ to facilitate earlier diagnosis and treatment, and to provide support in the treatment of DM. An additional issue is whether DM should be considered a cumulative dose-limiting toxicity. Although the ideal duration of STZ therapy is unknown, based on the analysis of Gompertzian growth kinetics in tumors by Norton premature discontinuation of effective chemotherapy would be expected to result in rapid tumor regrowth. Because all of the dogs that developed DM required insulin therapy, it seems unlikely that DM would be more severe with additional doses, and additional STZ therapy might result in improved tumor control.
Streptozotocin is a known nephrotoxin. In people, toxicity is dose-related and cumulative. Although reversible when mild to moderate in severity, fatal nephrotoxicity has been reported. In dogs, high doses cause necrosis of renal tubular epithelium. Given without fluid diuresis, doses used in this protocol cause proximal convoluted tubular injury (necrosis or vacuolization). Increases in BUN and serum creatinine concentrations are possible as are hematuria, glucosuria, and ketonuria. With a concurrent 7-hour diuresis protocol, 1 of 17 dogs treated with STZ every 3 weeks developed increased serum creatinine concentrations. This dog had preexisting evidence of renal disease (ie, granular casts) and thus may have suffered progression of existing disease or have been at greater risk for renal injury with STZ. Using the same diuresis protocol with close monitoring of serum creatinine concentrations, no abnormal values were observed in this study. However, 2 dogs developed clinically relevant renal abnormalities suspected to be a consequence of STZ therapy, namely Fanconi syndrome and nephrogenic diabetes insipidus. Although these conditions never resolved, both dogs had acceptable quality of life and were euthanized for progressive insulinoma. The increased dose intensity may have resulted in greater nephrotoxicity, but this cannot be determined from the present study. Based on the development of presumptive irreversible dose-limiting nephrotoxicity in 2 of 19 dogs, clinicians should consider evaluation of indicators of renal tubular injury in addition to serum creatinine concentration, including proteinuria, glucosuria, cylindruria, presence of renal epithelial cells in the urine sediment, and low specific gravity (although this may be confounded by DM or prednisone therapy) before administering STZ. For dogs with abnormal results, delay of treatment and further diagnostic evaluation is indicated.
Macromolecule alkylation is an important mechanism by which drugs injure the liver. Streptozotocin is a recognized hepatotoxin, associated with reversible increases in liver enzyme activity in people.[17, 19] In dogs, STZ administration results in increases in serum alkaline phosphatase, ALT, and aspartate aminotransferase activities, and, at high doses, increased serum bilirubin concentrations. In this study, evaluation of liver enzyme activity was confounded in some dogs by concurrent use of prednisone or hepatic metastasis. Five dogs experienced increases in liver enzyme activity suspected to be associated with STZ. Because bile acid concentrations were not evaluated in these dogs, it is unknown whether or not liver dysfunction was present. Toxicity studies in dogs using a variety of doses of STZ identified hepatocellular cytoplasmic vacuolization at all doses, including the range used in this study. Only 1 dog with increased liver enzyme activity in the current study underwent biopsy. Vacuolar hepatopathy was diagnosed. If, and at what cumulative dose of STZ or degree of increased ALT activity, irreversible liver injury would occur is unknown. A specific recommendation regarding an acceptable cut-off for continuation of STZ in the face of increased ALT activity cannot be made based on the results of this study. We believe that discontinuation of treatment should be considered. A recent study demonstrated the efficacy of Denamarin (S-adenosylmethionine and silybin A+B [phosphatidylcholine complex])9 in preventing or ameliorating increased liver enzyme activity associated with another nitrosourea agent, CCNU (lomustine).10, Denamarin might also be a consideration for dogs undergoing STZ treatment. It is not known whether this nutraceutical decreases the efficacy of alkylating agents.
Two dogs collapsed because of hypoglycemia at the end of the STZ infusion. STZ may have induced degranulation of insulinoma and pancreatic β cells resulting in severe hypoglycemia. Degranulation of pancreatic β cells after administration of STZ has been demonstrated histologically in dogs, and STZ-associated weakness has been reported in another dog. It is important to monitor dogs and be prepared to treat this adverse effect, because it has been fatal in people. In addition, because of this potential toxicity, clinicians should consider allowing access to food during the STZ infusion.
The use of STZ in managing insulinoma in dogs is controversial. Because this study did not include a control group, the impact of biweekly STZ on survival cannot be determined. Reported STs of dogs with insulinoma have been variable, but with treatment long term survival is possible.[4-8] Surgical excision by partial pancreatectomy and excision of resectable metastatic lesions is the primary therapy for dogs with insulinoma and results in longer survival than medical management.[1, 4, 9] Median ST after surgery is 12–15 months, with the majority of dogs dying of recurrence of hypoglycemia and clinical signs.[1, 4, 5, 9] Recently, improved ST (median, 26 months) was reported in 19 dogs treated surgically, suggesting that surgery as a sole modality may be adequate for managing some dogs with insulinoma. Because this study is more recent than other studies of surgery for insulinoma, earlier detection and treatment, combined with improved surgical technique and postoperative management, likely contributed to longer survival. In addition, 14/19 dogs had stage I tumors and dogs with nonresectable primary tumors were not included in the analysis, and thus case selection was also a factor. In the same study, the median ST of dogs treated with medical management was 452 days; however, for dogs receiving only medical management (due to metastasis or nonresectable primary tumors) the MST was only 196 days, supporting the need for better therapeutic options for this group of dogs.
Streptozotocin-based protocols have been the standard antitumor therapy for humans with metastatic or nonresectable insulinomas. These protocols are associated with response rates of 6–69% (influenced by definition of response as resolution of hypoglycemia versus response of measureable disease) and a long time to maximal response (4 months) for pancreatic endocrine tumors including insulinoma.[21, 22] Moore et al described measurable responses in 2 dogs with presumptive metastatic insulinoma; however, the presence of metastatic tumor was not confirmed in the lymph nodes and liver masses used to measure response. Two dogs had resolution of polyneuropathy. The median duration of normoglycemia for dogs with stage II and III disease was 163 days, superior to that previously reported in dogs with stage II and III insulinoma treated with surgery (<3 months), but similar to their historical controls. Bell et al reported resolution of hypoglycemia in a dog treated with STZ.
Response rate to STZ cannot be determined from the results of this study. Similar to the dogs in the Moore study, all of the dogs presented here had metastatic or nonresectable insulinoma (as determined by surgeon assessment at exploratory laparotomy) or metastasis confirmed histologically or suggested by persistent hyperinsulinemia, hypoglycemia, or both. Median PFS and survival were 196 and 308 days, respectively, but are confounded by symptomatic therapy, use of other cytotoxic therapies, and owner decision to euthanize. Consequently, this study enhances our understanding of toxicity associated with STZ in dogs, but additional work is needed to define the role of STZ in the treatment of insulinoma in dogs. Although STZ is a tolerable chemotherapeutic agent in dogs when given with saline diuresis, serious toxicity is possible. A prospective study comparing dogs with metastatic or nonresectable insulinoma undergoing surgery with and without adjunctive STZ would be needed, and stratification by known prognostic indicators including TNM stage, postoperative hypoglycemia,[1, 3, 7] and Ki-67 index would be important to identify populations that would benefit from STZ therapy. Based on the results of this study, it is important to monitor patients receiving STZ for increased liver enzyme activity, renal injury, and DM. Owners should be counseled about potential adverse effects of STZ.
We thank Drs Courtney Zwahlen, Michelle Wall, and all of the clinicians involved in the management of the cases included in this study. We also thank Dr Deborah Keys for statistical analysis. This study was partially funded by the UGA CaRES (Cancer Education, Research, and Service) for Pets Fund.
Conflict of Interest Declaration: Authors disclose no conflict of interest.
Zanosar; Teva Parenteral Medicines, Irvine, CA
Torbugesic; Fort Dodge Animal Health, Fort Dodge, IA
Anzemet; Sanofi-aventis US, LLC, Bridgewater, NJ
Metoclopramide injection (generic); Hospira Inc, Lake Forest, IL
Cerenia; Pfizer Animal Health, New York, NY
SAS 9.2; SAS, Cary, NC
Metabolic Genetic Disease Screening Program, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA