Radiolabeled somatostatin analogues, a form of peptide receptor radionuclide therapy, have gained clinical use in treating neuroendocrine tumors found in the midgut, the non-midgut, and the pancreas. NETs generally have high expression of somatostatin receptors, the target of somatostatin analogues. The most commonly used radionuclides, lutetium 177 and yttrium 90 are β emitters.
Thorvardur Halfdanarson, MD
Radiolabeled somatostatin analogues (SSAs), a form of peptide receptor radionuclide therapy (PRRT), have gained clinical use in treating neuroendocrine tumors (NETs) found in the midgut, the non-midgut, and the pancreas. NETs generally have high expression of somatostatin receptors, the target of SSAs. The most commonly used radionuclides, lutetium 177 (177Lu) and yttrium 90 (90Y), are β emitters.
The differences between the 2 radionuclides are demonstrated in energy, tissue penetration, and half-life, Thorvardur Halfdanarson, MD, told audience members during the Physicians’ Education Resource®, LLC (PER®),4th AnnualSchool of Gastrointestinal Oncology™ (SOGO®) meeting, held April 27, 2019, in Washington, DC.1
“Keep in mind that the energy level emitted by lutetium is lower than yttrium, tissue penetration is shorter for lutetium, and the half-life is longer for lutetium,” Halfdanarson said. This has an effect on “toxicity and possibly efficacy,” he added.
Retrospective studies2of177Lu and90Y suggest that complete responses are very rare and range from 0% to 6%. However, partial responses are much more common and range from 4% to 38%. Progression-free survival (PFS) ranges from 16 months to 33 months, and overall survival (OS) ranges from 22 months to 46 months. “This is a mixed bag of retrospective studies with different types of patients,” cautioned Halfdanarson, a hematologist/oncologist in the Department of Oncology (Medicine) at Mayo Clinic in Rochester, Minnesota. “What we need is a large prospective trial.”
Strosberg J et al3followed 229 patients with metastatic midgut NETs, of whom 116 were randomized to receive177Lu-Dotatate at a dose of 7.4 gigabecquerel plus best supportive care (including 30 mg octreotide long-acting repeatable [LAR]) every 8 weeks and 113 to receive 60 mg octreotide LAR alone every 4 weeks (control). PFS was assessed every 12 weeks. There were 105 patients (91%) in the lutetium arm and 101 patients (89%) in the control arm with grade 3/4 uptake of radiotracer via scintigraphy (with higher grades indicating greater somatostatin receptor expression). “As of the publication date of the study, PFS was not reached in the experimental arm, but it has been estimated to be 28.4 months,” Halfdanarson said. “For patients taking octreotide [control], PFS was reported as 8.4 months.”
Fourteen deaths were reported in the interim analysis of OS in the lutetium group, and 26 deaths were reported in the octreotide group (P= .004). The investigators reported objective response rates (ORRs) of 18% in the lutetium arm and 3% in the control arm (P<.001). In the lutetium arm, the most common all-grade adverse events (AEs) reported were nausea (59%) and vomiting (47%); grade 3/4 AEs were nausea (4%) and vomiting (7%). In the octreotide arm, all-grades AEs were nausea (12%) and vomiting (10%), with grade 3/4 AEs of nausea (2%) and vomiting (1%) reported. “Grade 3/4 hematologic toxicities were thrombocytopenia (2%), lymphopenia (9%), leukopenia (1%), and neutropenia (1%) [in the lutetium arm],” Halfdanarson said.
“Our concern is with dose treatment delays as patients show up at the clinic with platelet counts of 49 or lower,” Halfdanarson said. “As a result, we cannot administer the drug, which costs $53,000. We can’t send the drug back to the manufacturer because the drug is actively decaying.” One way to avoid treatment delays is to have the patient come into the clinic a week to 10 days before administration for a complete platelet count. “At least we can cancel the shipment of the drug if the patient’s count is low,” he noted.
There are very few data available discussing retreatment with PRRT, but one retrospective study4followed patients who had PFS ≥18 months after initial PRRT. Retreatment of 168 patients resulted in an ORR of 15.5%, and re-retreatment of 13 patients produced an ORR of 38.5%. The investigators reported that hematologic toxicity (including myelodysplastic syndromes [MDSs] and acute myeloid leukemia [AML]) and renal toxicity did not appear to be increased as a result of retreatment.
Halfdanarson noted that a benefit of re-retreatment in terms of cumulative OS was observed in subsets of patients with bronchial/gastroenteropancreatic and midgut NETs (P<.01 for both). Although the study authors4noted a trend toward OS improvement among patients with pancreatic NETs, it did not reach statistical significance (P= .57).
Hematologic toxicities associated with PRRT have been relatively minor, with grade 3/4 AEs (other than lymphopenia) resolved within 3 months for 77% of patients in one study.5The most severe complication of PRRT, however, is delayed toxicity, specifically MDS/AML.
Brabander et al5reported that 9 patients (1.5%) developed MDS a median of 28 months after treatment, and 4 patients (0.7%) developed acute AML a median of 55 months after treatment. “This is a horrible complication to have because the survival with treatment-related AML is very short,” Halfdanarson said. Grade 3/4 nephrotoxicity and hepatoxicity were minor. Brabander and colleagues reported that 6 patients (1%) developed renal failure during follow-up, but all cases were attributable to other causes.
“MDS and AML are probably more common than we originally thought,” said Halfdanarson, “so keep an eye on it.” MDS and leukemia occurred in 2% of patients a median of 8 years after NET diagnosis in one study,6“but a lot of these patients would not have lived 5 years if they did not receive PRRT,” he said.
The clinical scenarios for PRRT vary. The treatment appears safe for central nervous system and orbital metastases. In patients with extensive bone metastases, worsening pain has been reported after PRRT, but steroid therapy may provide relief. Worsening of carcinoid crisis has been reported.177Lu is predominantly excreted in the urine, with a half-life of 3.5 hours for elimination in blood. In general, negligible radioactivity is present in body fluids, with the exception of urine.1
Future research involving PRRT will include use in the neoadjuvant setting, in combination with chemotherapy (most likely capecitabine or temozolomide), somatostatin antagonists, targets other than somatostatin receptors, and α emitters.1