Partial tachyphylaxis to somatostatin (SST) analogues in a patient with acromegaly: the role of SST receptor desensitisation and circulating antibodies to SST analogues.

OBJECTIVE: Somatostatin (SST) analogues are a key option in the management of a variety of conditions, including acromegaly. Tachyphylaxis to SST analogues is not documented in acromegaly. We describe such a phenomenon. DESIGN AND METHODS: A 74-year-old female with acromegaly previously treated with (90)Y implant, external radiotherapy and thrice daily s.c. octreotide had stable GH levels of 19 mU/l. GH progressively rose following switches to lanreotide and depot octreotide as Sandostatin LAR: from 29 to 126 mU/l. Magnetic resonance imaging and (111)In-pentetreotide scanning revealed no tumour growth or alteration in SST receptor (SSTR) status. Tachyphylaxis to SST analogues was considered. Therapy was discontinued and re-introduced in daily 200 microg/24 h increments by continuous s.c. infusion, to a maximum of 1000 microg/24 h, and maintained over 3 weeks with daily, followed by weekly, GH profiles. Competitive (125)I-octreotide radioligand binding assays measured in vitro bio-activity of anti-SST analogue antibodies. In vitro SSTR binding studies utilised SSTR-expressing rat cortex membrane. RESULTS: Median GH fell by 93% from 504 to 39.5 mU/l and rose reproducibly on continued infusion to 120 mU/l. Octreotide withdrawal for 16 h produced a 64% increase in sensitivity. High-affinity IgG anti-lanreotide (IC(50)=187 pmol/l) and anti-octreotide (IC(50)=82 nmol/l) antibody, with no crossreactivity with natural SST, was demonstrated. In vitro inhibition of (125)I-octreotide SSTR binding by anti-SST analogue crossreacting antibody was observed at 1:1 serum dilution. CONCLUSIONS: This is the first report of tachyphylaxis to SST analogues in acromegaly. We believe that the short time course of resensitisation following acute octreotide withdrawal is suggestive of an effect(s) on receptor function or on the receptor signal transduction cascade at sites further downstream, rather than an immune-mediated phenomenon.

Occurrence and effects of octreotide antibodies during nasal, subcutaneous and slow release intramuscular treatment.

OBJECTIVE: Previous studies have indicated that antibody formation against octreotide is extremely rare. We examined the occurrence of octreotide antibody formation after treatment with three administration forms in large populations of patients with acromegaly or carcinoid syndrome. DESIGN: (i) Nasally administered octreotide: 70 previously untreated patients and 81 previously s.c. octreotide-treated patients participated. (ii) Subcutaneously administered octreotide: 172 acromegalic patients and 59 patients with carcinoid syndrome treated for up to 12 years participated. (iii) Intramuscularly administered depot octreotide (Sandostatin LAR): 62 acromegalic patients participated. METHODS: Presence of antibodies is defined as increased precipitation by polyethylene glycol of (125)I-octreotide after incubation with serum; this was also used for screening of cross-reaction with somatostatin and lanreotide (Somatuline). RESULTS: In patients who received nasal octreotide for at least 9 and up to 12 months (n=42), the occurrence of octreotide antibodies was 77% and 81% for previously untreated and treated patients respectively. In subcutaneously treated patients it was 63/231 (27%) after a mean exposure of 3 years. In patients treated for more than 5 years (n=53) it was 57% and after 8 years (n=18) 72%. In contrast, no patient could with certainty be identified to be antibody-positive after a mean of 2.5 years intramuscular Sandostatin LAR treatment (n=47). In all populations, the antibody-positive patients were as well controlled as the antibody-negative patients. Octreotide antibodies did not cross-react with native somatostatin (n=141), while about 25% of the antibody-positive sera did cross-react with the somatostatin analogue, lanreotide (Somatuline, Ipstyl, Angiopeptin). CONCLUSIONS: Antibody formation against octreotide is much more frequent than previously believed. It depends primarily on drug exposure time and route of administration. It does not alter the GH/IGF-I status in treated acromegalic patients and induces only mild local reactions in some patients.

A case of antibody formation against octreotide visualized with 111In-octreotide scintigraphy.

A case of antibody formation in a patient with carcinoid syndrome is described. The patient was treated with octreotide in dosages up to 1.5 mg/day. Serum samples were analysed for the presence of octreotide antibodies before and after 20 months of octreotide treatment. In-vivo 111In-octreotide scintigraphy was performed before and during therapy, and after antibodies had developed. Before treatment, no serum antibodies against octreotide were detected. After 20 months of treatment, they were detectable up to a 1:115 serum dilution. The serum binding of 125I-Tyr3-octreotide was blocked by adding excess unlabelled Tyr3-octreotide, indicating the presence of specific octreotide antibodies. Before treatment, a normal distribution of radioactivity in the spleen and kidneys, irregular uptake in the liver due to metastases, and a hot spot in the lower abdomen were found during 111In-octreotide scintigraphy. After antibodies had developed, increased radioactivity over the heart and high background radioactivity in the abdomen with only faint visualization of the spleen, liver, and kidneys were found, indicating a prolonged presence of 111In-octreotide in the blood resulting from its being bound to antibodies. Increased radioactivity was also seen at the injection sites of the drug in the upper legs. In-vitro incubation of biopsy tissue from this site with 125I-Tyr3-octreotide revealed diffuse guanosine triphosphate (GTP) independent specific binding, indicating non-G-protein linked binding of labelled octreotide. This report describes the characteristic abnormalities during in-vivo 111In-octreotide scintigraphy in a patient with octreotide antibodies.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term efficacy and tolerability of octreotide treatment in acromegaly.

Twenty-five acromegalic patients were studied during 6 years of treatment with octreotide, with a particular focus on the following parameters: (1) Administration schedule: in 10 patients, continuous subcutaneous (SC) octreotide infusion was compared with injections of octreotide at three dose levels (100, 250, and 1,500 micrograms/24 h) and was found to induce a greater and less-fluctuating 24-hour growth hormone (GH) suppression. (2) Carbohydrate tolerance: average 24-hour blood glucose levels were unaffected by octreotide, regardless of administration schedule. Oral carbohydrate tolerance and intravenous (IV) glucose tolerance were unaffected by continuous octreotide infusion. However, octreotide injection given shortly before the tests reduced carbohydrate tolerance. (3) Thyroid function: octreotide and somatostatin acutely reduce the response of thyroid-stimulating hormone (TSH) to thyrotropin-releasing hormone (TRH). After a few days of treatment, it was demonstrated that octreotide slightly inhibits iodothyronine deiodination and induces a transient reduction in serum triiodothyronine (T3), rapidly compensated for by a persistent slight elevation of serum TSH. (4) Fat absorption was estimated as 24-hour fecal fat content and found to be in the same high-normal range before and after octreotide treatment. Vitamin K and D absorption were unaffected by octreotide. The incidence of gallstone formation was not greater than in the general Danish population, possibly due to the schedule used for octreotide injections. (5) Foot volume was regularly estimated and found to decrease with time, on average by 12% during the first 18 months.(ABSTRACT TRUNCATED AT 250 WORDS)

Radiolabeled octreotide. What lessons for antibody-mediated targeting?

Octreotide is a synthetic analog of the peptide hormone somatostatin (SMS). A wide variety of tumors express enhanced numbers of SMS receptors, notably neuroendocrine tumors and lymphomas, but also some of the more common adenocarcinomas. Octreotide contains only eight amino acids, some of which are in the (D) configuration in order to enhance the stability of the molecule in vivo. Tyrosine and DTPA-containing analogs of octreotide have been synthesized and labeled with iodine-123 and indium-111, respectively, with the intention of targeting SMS receptor-containing tumors for diagnostic purposes. Both radiopharmaceuticals demonstrate a high sensitivity and specificity for these tumors, indicating a clinical role for these agents in management of these diseases. Lessons can be learned from the success of these agents when designing improved antibody-based molecules. Tumor uptake of radiolabeled octreotide is very rapid, occurring within minutes of administration. Blood clearance is also rapid, such that tumors are soon visible even in areas of high blood background. An interesting finding has been the differences between the pharmacokinetics of the iodinated and indium-labeled species. Although the majority of 123I-Tyr3-octreotide undergoes hepatobiliary excretion, 111In-DTPAPhe1-octreotide is eliminated predominantly by the kidneys. These results suggest that the smallest possible antibody-like tracers are likely to have advantages over native immunoglobulins and conventional Fab-like fragments.

Effects of antibodies against octreotide in two patients with acromegaly.

Two patients developed specific IgG antibodies against octreotide after 2-3 years' treatment for acromegaly with this long acting somatostatin analogue. The presence of these antibodies reduced the plasma disappearance rate of total extractable octreotide by 60 and 80% respectively. When compared to that of non-immune acromegalic patients, the plasma half-life of octreotide in these two patients was 300 and 450 vs 110 min in those with no detectable octreotide antibodies. The sole observed consequence of the immunization was a marked prolongation of the interval of maximum GH inhibition from a mean of 5 to 8 and 10 h in the two patients described after octreotide injection.

Lack of antibody formation during long-term subcutaneous treatment with the somatostatin analogue octreotide in acromegaly.

Serum samples from 13 patients with active acromegaly on long-term sc treatment with octreotide (SMS 201-995, 1-36 months, mean daily dose 285 micrograms) were taken 12 h after the injection of their regular evening doses. Octreotide assay was performed using 125I-Tyr-SMS and a polyclonal rabbit anti-serum. For assessment of antibody formation both serum coated charcoal adsorption (adsorption of free octreotide) and polyethylene glycol precipitation (precipitation of IgG complexes) were used. The mean binding percentage in the patients proved to be similar to that of 5 healthy volunteers (p greater than 0.10). No specific binding was detected, whatever method used. No correlation was found between the binding percentages and octreotide serum levels, duration of octreotide treatment or daily octreotide dose (p greater than 0.10). These results strongly suggest that clinically relevant endogenous antibody formation is not a frequent event during long-term sc treatment of acromegalic patients with octreotide.

Chronic administration of the somatostatin analogue SMS 201-995 does not lead to endogenous antibody formation.

Plasma samples of seven patients with gut and pancreatic endocrine tumours who have been on long-term treatment with a long-acting somatostatin analogue (SMS 201-995) were investigated for endogenous antibodies to the peptide by incubation with radiolabelled SMS 201-995. The duration of treatment with the somatostatin analogue was between 9 and 26 months and the dose from 100 to 300 micrograms day-1. In none of the patients could antibodies to SMS be detected. The effect of this somatostatin analogue is unlikely to be impaired by formation of endogenous antibodies, even after long-term treatment.