Treatment of Gastrin-Secreting Tumor With Sustained-Release Octreotide Acetate in a Dog

Introduction

A gastrin-secreting tumor is a rare, malignant neuroendocrine tumor in veterinary medicine. An excessive gastrin secretion can lead to hydrochloric acid hypersecretion resulting in anorexia, vomiting, and diarrhea.¹ Systemic chemotherapy together with gastroprotectant and gastric acid secretion inhibitors are commonly used to treat these symptoms.^1,2 Somatostatin impedes hormone release and growth of hormone-secreting cells via binding to its receptors. Somatostatin receptor is known to be expressed at high levels in human hormone-secreting tumor cells, and somatostatin analogues, such as octreotide, have been used to treat humans with carcinoid.^2,3 Octreotide is a potent agonist of somatostatin receptor (SSTR) 2 that is reported to have antitumor activity against neuroendocrine tumors.⁴ However, half-life of octreotide within the blood is approximately 2 hr; thereby, human patients require frequent administration (2 or 3 times daily).²

In a dog with pancreatic gastrinoma, plasma somatostatin levels decreased by half at 2 to 4 hr of octreotide administration.⁵ In contrast, sustained-release formulation of octreotide acetate, can be more conveniently administered monthly for human carcinoid patients. Octreotide acetate consists of microspheres of poly-DL-lactide-co-glycolideglucose containing octreotide. Slow release of octreotide within the microspheres occurs as the polymer biodegrades, primarily through tissue fluid hydrolysis. Approximately 2 wk are required for the equilibrium blood concentration of octreotide to be achieved in people. The effective level of octreotide persists for 4 wk after the second dosage. In healthy beagles, microsphere-encapsulated lanreotide, another long-acting somatostatin analogue similar to sustained-release formulation of octreotide acetate^a, maintained effective blood concentration for over a mo.⁶ In human cases, monthly administration of sustained-release formulation of octreotide acetate^a is equally effective as three-times daily administration of regular octreotide acetate^f in the treatment of neuroendocrine tumor.⁴ In a study of 93 human patients with neuroendocrine tumors and carcinoid syndrome treated with sustained-release formulation of octreotide acetate^a, 66% of the patients showed complete or partial improvement of the symptoms.⁷ A more recent placebo-controlled study of a human with neuroendocrine tumors showed that the median time to progression was significantly prolonged in the sustained-release formulation of octreotide acetate^a-treated group (14.3 mo) compared to the placebo-treated group (6 mo).⁸ In contrast, the information of the use of octreotide for treatment of neuroendocrine tumors is scant in the veterinary literature; the use of sustained-release formulation has especially not been described in dogs.^5,9 The purpose of this case report is to describe the hormonal response to treatment with sustained-release formulation of octreotide acetate^a in a dog with gastrin-secreting tumor.

Case Report

An 8 yr old, intact male Shiba Inu was referred to Hokkaido University Veterinary Teaching Hospitalfor evaluation of loose stool and polydipsia of 1 mo duration and hematuria, vomiting, and anorexia of 1 wk duration. Upon presentation (day 1), the dog was bright, alert, and responsive. The dog weighed 10.3 kg with body condition score of 2 out of 5. No remarkable abnormality was found on the physical examination except for grade II/VI left-sided systolic cardiac murmur.

A complete blood count and serum chemistry profile revealed no clinically relevant abnormalities. On electrocardiogram, second-degree atrioventricular block and sporadic premature ventricular contraction were detected. No abnormalities were detected on 3-view thoracic radiographs. Two-view abdominal radiographs were unremarkable except for mild microhepatica. On abdominal ultrasonography, numerous well-circumscribed nodules were detected in the hepatic parenchyma, which distributed diffusely to all lobes of the liver. These nodules were round-shaped, ranging from 3 to 11 mm in diameter, with hyperechoic central portion and hypoechoic peripheral region, giving them a “target-like” appearance (Figure 1A). There was also a 7 mm hypoechoic nodule in the parenchyma of the right limb of the pancreas. Screening of the rest of the abdominal structures including the gastrointestinal tract revealed no further abnormalities. To further characterize the nodules in the hepatic parenchyma, contrast-enhanced ultrasonography using an intravenous bolus of a second-generation contrast agent^b was performed, which showed characteristic contrast influx in the arterial phase with rapid washout time and almost complete contrast defect in the portal and parenchymal phase (Figure 2A, B). These characteristics were interpreted as consistent with malignant neoplasia with special suspicion for being neuroendocrine neoplasia.¹⁰ Ultrasound-guided fine needle aspiration of the hepatic nodules was performed. The cytologic examination of the sample revealed numerous epithelial cells with round to ovoid shape, frequently appearing as clusters of scattered bare nuclei, consistent with the diagnosis of neuroendocrine neoplasia. From these findings, tentative diagnosis of primary diffuse hepatic carcinoid or pancreatic neuroendocrine neoplasia with wide-spread metastasis to the hepatic parenchyma was made.

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On day 6, exploratory laparotomy was performed; multiple white- to pale-colored nodules were identified throughout the all lobes of the liver. No nodular lesion was identifiable by gross inspection and manual palpation of the pancreas. One of the hepatic nodules was excised for histopathological evaluation; pancreatic biopsy was not performed due to lack of gross abnormality. The other organs, including the stomach and intestine, did not show any abnormalities. The dog recovered from the anesthesia unremarkably and was discharged the day after the surgery. On day 8, the dog was presented to the hospital for evaluation of acute onset of severe continuous vomiting. A paraneoplastic gastrointestinal ulceration secondary to possible hypergastremia (Zollinger-Ellison syndrome) was suspected, and treatment with intravenous administration of omeprazole^c (1 mg/kg) and intravenous fluid^d administration was initiated, which dramatically resolved the symptom. The dog was discharged with omeprazole^c (1 mg/kg per os [po] q 24 hr) and famotidine^e (1 mg/kg po q 24 hr). The histopathological diagnosis was neuroendocrine neoplasia (Figure 3A); additional immunohistochemical stains for chromogranin A and gastrin revealed that the neoplastic cells were positive for these two substances, leading to a diagnosis of gastrin-secreting neoplasia (Figure 3B).

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To suppress the secretion of gastrin, treatment with somatostatin analogue, octreotide acetate^a, was considered; however, the product was cost-prohibitive and required repeated subcutaneous injections. Alternatively, use of sustained-release octreotide acetate^a was selected. To confirm that the patient actually had hypergastrinemia that was responsive to octreotide acetate administration, serum concentration of gastrin was measured on day 21, before and 2, 4, 6, and 8 hr after subcutaneous administration of regular octreotide acetate^f (8 μg/kg).⁴ The baseline serum gastrin level was substantially elevated (45,613 pg/mL) in the case compared with those of the healthy beagles concurrently measured as controls (n=3, 204±22 pg/mL [mean±standard diviation]). The gastrin level decreased to approximately half of the baseline with the administration of octreotide acetate and returned to near-baseline level in 8 hr (Figure 4A).

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Treatment with sustained-release formulation of octreotide acetate^a was started on day 33. The dosage and treatment interval was chosen empirically (0.5 mg/kg, q 28 days), extrapolating protocol used for humans with carcinoid.⁵ At this time, the dog had loose stool and intermittent vomiting, but was still maintaining normal attitude and appetite. Two of the nodules that were highly-reproducible were selected as target lesions and the size of these nodules was remeasured (all ultrasonography examination was done by the same clinician) on day 56 and 173 (23 and 140 days after initiation of sustained-release formulation of octreotide acetate^a, respectively) by repeated abdominal ultrasonography, which showed that the size of the tumor was unchanged or slightly decreased on day 56 but significantly increased on day 173 (Figure 1B). The serum gastrin level was measured weekly to determine the degree and duration of the effect of sustained-release formulation of octreotide acetate^a (Figure 4B). The serum gastrin level gradually decreased to half the baseline level on day 65 (32 days after initiation of sustained-release formulation of octreotide acetate^a), but did not normalize, after which it progressively increased in parallel with the increase in tumor volume. Since the effect of sustained-release formulation of octreotide acetate^a at our starting dose was incomplete, a doubled dose (1 mg/ kg) was administered on day 86 (53 days after initiation of sustained-release formulation of octreotide acetate^a) with no apparent effect on serum gastrin concentration. Clinically, the dog continued to have intermittent diarrhea and vomiting, but still maintained normal activity level and good appetite; the dog's body weight gradually increased from 10.0 kg on day 21 to 10.7 kg on day 84. The dog's condition started to deteriorate slightly on day 140, showing mild lethargy and mild anorexia. Abdominal ultrasonography revealed further progression of the disease; increased size and number of the hepatic nodules were noted. Medical management was continued with shortened dose interval (from q 24 hr to q 12 hr) of omeprazole^c and famotidine^e; later, metoclopramide^g (0.5 mg/kg po q 8 hr) was added. On day 196, the case appetite severely decrease and became lethargic. For recovery of appetite, prednisolone^h (0.25 mg/kg po every other day) was prescribed. At this time, loss of weight was significant (9.3 kg) and serum gastrin level was significantly increased (gastrin concentration 95,222 pg/mL). On day 224, although the dog's activity and appetite were improved, a marked progression of hepatic nodules was apparent, accompanied by persistent elevated plasma gastrin level. Systemic chemotherapy was attempted to attenuate tumor growth. Based on the highly elevated gastrin level and the patient's symptoms, it was thought that standard chemotherapy would worsen the gastric mucosal injury, and metronomic chemotherapy with low-dose cyclophosphamideⁱ (15 mg/m² po q 24 hr) was chosen. However, the dog immediately developed severe vomiting, anorexia, and lethargy within a few days of treatment. Upon discontinuation of cyclophosphamideⁱ, these symptoms were reversed. Sustained-release formulation of octreotide acetate^a administration was continued every 4 wk for 10 doses with the last dose given on day 280 (247 days after initiation of sustained-release formulation of octreotide acetate^a); at that time, dog's body weight had decreased to 85% of the pretreatment level (8.5 kg). Medical management with omeprazole^c, famotidine^e, metoclopramide^g, and sucralfate^j was continued with waxing and waning clinical signs, but the dog's body weight steadily decreased until day 510 (477 days after initiation of sustained-release formulation of octreotide acetate^a), when the dog was lost to follow-up, suspected to have died at home.

Discussion

Serum gastrin level decreased with a test dose of regular octreotide acetate^f in the current case to approximately half of its original level, although this gastrin concentration was still far higher than that of controls. This finding was consistent with the previous report where subcutaneous injection of 4 or 8 μg/kg of octreotide approximately halved the serum gastrin concentration in a dog with gastrin-producing tumor, which suggested that the inhibitive effect of octreotide against gastrin secretion may not be as marked as reported in humans.^1,4 Human neuroendocrine tumors have been reported to frequently express SSTR 2; in a study of 33 human gastrinoma cases, the expression of SSTR 2, 3, 5, and 1 was noted in 100, 79, 76, and 30% of the cases, respectively.^11,12 Octreotide possesses high affinity to SSTR 2 and decreased affinity to SSTR 5, 3, 1, and 4, in this order.¹³ The drug exhibits inhibitory effect against hormonal secretion via binding to SSTR 2 and 5.³ A previous immunohistochemical study has shown that canine gastrinoma cells also express SSTR, and activation of SSTR 2 can inhibit gastrin and gastric acid production in normal dogs.^5,14 These lines of evidence suggest that octreotide could have a therapeutic effect as hormone suppressor in canine gastrin producing tumors; however, the affinity of octreotide to canine SSTR has not been evaluated, and whether octreotide binds to canine SSTR in a similar manner to human SSTR warrants further investigation.

Sustained-release formulation of octreotide^a caused a similar degree of gastrin decrease as the regular octreotide in the present case. The effect of sustained-release formulation of octreotide acetate^a was not immediate and the serum gastrin level gradually decreased. This lack of rapid effect could be explained by the slowly releasing manner of the drug. However, gastrin suppressive effect waned after the second dose, after which the serum gastrin concentration steadily increased. Possible reasons for this short-lasting effect may include the progression of the primary disease, decreased sensitivity of the tumor cells to octreotide, or both.¹⁵ Repeated abdominal ultrasonography showed that the tumor size increased after the second dose of octreotide, and it is possible that this progression caused further increase of gastrin secretion, which negated the effect of octreotide. Long-term use of octreotide is also known to cause decreased sensitivity of SSTR to the effect of octreotide due to down-regulation of SSTR expression as noted in humans.¹⁵ The expression level of SSTR was not investigated in the present case and whether the above phenomenon occurs in dogs is unknown.

The appropriate dose of sustained-release octreotide^a could not be determined from the present case. A dose of 5 mg (approximately 0.5 mg/kg of body weight) resulted in a similar degree of gastrin suppression to a test dose of regular octreotide (8 μg/kg); however, the reduction of serum gastrin was achieved for only a transient period and the degree of reduction was incomplete. A doubled dose (10 mg, or approximately 1 mg/kg) given later in the course of disease did not seem to be effective. Although it is possible that a higher dose given in the early stage of the disease could have been more effective, the data from other studies in humans and dogs suggest against this speculation.^1,5 A previous study in a dog with gastrinoma found that a subcutaneous administration of 8 μg/kg of regular octreotide^f caused the similar degree of gastrin inhibition compared to a dose of 4 μg/kg of the drug, even though much higher serum octreotide concentrations were achieved with the higher dose.⁵ Also, increasing the dose of octreotide has not been associated with an improvement of outcome in human patients; in previous studies involving patients with metastatic neuroendocrine tumors treated with octreotide, increasing daily octreotide dose by 2.5- to 3-fold did not appear to increase the tumor response rate.^16,17 The serum octreotide concentration could not be measured in the current case, which presents a major limitation for data interpretation; however, from the duration of slow-release of this product in humans (approximately for 4 wk) and from the fact that the serum gastrin concentration gradually decreased over 4 wk in this case, it was likely that the injected octreotide was released to the systemic circulation over a 4 wk period.

In the present case, the size of the primary tumor appeared stable until day 56, which may suggest that octreotide inhibited the growth of the tumor as well. Octreotide is known to suppress the growth of the tumor cells in human neuroendocrine tumors.¹⁸ The mechanism of action of octreotide for cell growth inhibition is also shown to be through activation of SSTR; ligand-mediated activation of SSTR not only inhibit hormone production but also suppress the cell proliferation and promote cellular apoptosis via inhibition of phosphorylation of adenylate cyclase and other second messenger kinases, thus blocking the signaling cascades for proliferation and survival.^4,19,20 Activation of SSTR 1, 2, 3, and 5 is known to induce the cell cycle hold, and activation of SSTR 2 and 3 is reported to induce apoptosis.²⁰ This inhibitive effect of octreotide for tumor growth has not been reported in canine gastrinoma, though. In the present case, sustained-release formulation of octreotide acetate^a was continued after progressive disease occurred in an attempt to slow the tumor growth. Efficacy of this treatment was not able to be determined in this case. The clinical benefit rate of octreotide in humans with gastrinoma has been reported to be as high as 53% (47% demonstrating tumor stabilization and 6% inducing a decrease in tumor size), and the median duration of response has been 25 mo.¹⁸

Conclusion

This case report is the first to describe the effect of sustained-release formulation of octreotide in canine gastrin-secreting tumor. Sustained-release formulation of octreotide^a caused a similar degree of gastrin suppression with a single dose for approximately 4 wk compared to a test dose of regular octreotide. Although the true duration of the gastrin suppression was difficult to assess due to concurrent tumor progression after the initial stabilized period, the findings of the current case suggests that sustained-release formulation of octreotide^a is effective, at least in a short period, for hormonal suppression and possibly a tumor growth inhibition, and warrants further investigation as a therapeutic option for dogs with gastrin-secreting tumor and other neuroendocrine tumors.