Successful receptor-mediated radiation therapy of xenografted human midgut carcinoid tumour

Abstract

Somatostatin receptor (sstr)-mediated radiation therapy is a new therapeutic modality for neuroendocrine (NE) tumours. High expression of sstr in NE tumours leads to tumour-specific uptake of radiolabelled somatostatin analogues and high absorbed doses. In this study, we present the first optimised radiation therapy via sstr using [(177)Lu-DOTA(0)-Tyr(3)]-octreotate given to nude mice xenografted with the human midgut carcinoid GOT1. The tumours in 22 out of 23 animals given therapeutic amounts showed dose-dependent, rapid complete remission. The diagnostic amount (0.5 MBq [(177)Lu-DOTA(0)-Tyr(3)]-octreotate) did not influence tumour growth and was rapidly excreted. In contrast, the therapeutic amount (30 MBq [(177)Lu-DOTA(0)-Tyr(3)]-octreotate) induced rapid tumour regression and entrapment of (177)Lu so that the activity concentration of (177)Lu remained high, 7 and 13 days after injection. The entrapment phenomenon increased the absorbed dose to tumours from 1.6 to 4.0 Gy MBq(-1) and the tumours in animals treated with 30 MBq received 120 Gy. Therapeutic amounts of [(177)Lu-DOTA(0)-Tyr(3)]-octreotate rapidly induced apoptosis and gradual development of fibrosis in grafted tumours. In conclusion, human midgut carcinoid xenografts can be cured by receptor-mediated radiation therapy by optimising the uptake of radioligand and taking advantage of the favourable change in biokinetics induced by entrapment of radionuclide in the tumours.

Figure 1

Uptake of [¹⁷⁷Lu-DOTA⁰-Tyr³]-octreotate was studied in tumours as well as in several organs 24 h p.i. In the tumours, the maximum uptake was 17±3.3%IA g⁻¹ (mean±s.e.m. n=5 for all groups) and was seen in animals receiving 15 MBq (0.5 μg) and 7.5 MBq (0.25 μg) [¹⁷⁷Lu-DOTA⁰-Tyr³]-octreotate. Decreasing uptake for higher administered doses indicated saturation of sstr in tumour tissue. For the critical organs kidney and liver, no signs of saturation was seen. P-values reflect the difference vs 7.5 MBq.

Figure 2

The biokinetics of [¹⁷⁷Lu-DOTA⁰-Tyr³]-octreotate after a diagnostic amount (0.5 MBq) as well as after a therapeutic amount (30 MBq) was studied in two groups of animals injected with [¹⁷⁷Lu-DOTA⁰-Tyr³]-octreotate. In the animals receiving 0.5 MBq [¹⁷⁷Lu-DOTA⁰-Tyr³]-octreotate, the activity concentration declined over time. On the contrary, in animals receiving 30 MBq, the activity concentration was retained over time and the biokinetic curves differed significantly both at 7 days p.i. (P=0.00044) and 13 days p.i. (P=0.00081). The values are presented as mean±s.e.m., n=5 at all time points for both groups.

Figure 3

Tumour-to-normal tissue activity concentration ratio (TNC) of [¹⁷⁷Lu-DOTA⁰-Tyr³]-octreotate followed up to 13 days after a single injection of 30 MBq. At 3, 7 and 13 days p.i., the TNC value increased compared to the situation 1 day p.i. This indicates a favourable relation over time for the radiation to the tumour in comparison with the radiation to the critical organs liver, kidney and bone marrow. P-values reflect the difference vs 1 day p.i.

Figure 4

Treatment with [¹⁷⁷Lu-DOTA⁰-Tyr³]-octreotate resulted in a rapid, dose-dependent reduction of tumour volume. For animals receiving 7.5 and 15 MBq, the effect was moderate and temporary, whereas for animals receiving 30 MBq CR was achieved. In animals treated with 30, 60 or 120 MBq, 22 out of 23 animals had CR. The tumour volume presented is the mean volume±s.e.m. for each group, set to 1 at the start of the experiment. The P-values for the number of CR for animals treated with 30, 60 and 120 MBq vs controls are 0.00032, 0.0011 and 0.000014, respectively.

Figure 5

Typical morphological appearance of GOT1 tumours 1 day (B), 3 days (C), 7 days (D) and 13 day (E) after treatment with 30 MBq [¹⁷⁷Lu-DOTA⁰-Tyr³]-octreotate compared to an untreated tumour (A). After injection of [¹⁷⁷Lu-DOTA⁰-Tyr³]-octreotate, significant increases in the apoptotic cell count vs controls could be seen both 1 day p.i. (P=0.0000030) and 3 day p.i. (P=0.00000059). At 3 days p.i., all five tumours presented with large confluent necroses and oedema. At 7 and 13 days p.i., the apoptotic cell count decreased and the oedema and necroses were gradually replaced by fibrosis. The apoptotic cell count was 1.9±0.29 (mean±s.e.m.) for control animals and 26±4.3, 29±3.1, 1.2±0.65 and 0±0 at 1, 3, 7 and 13 days p.i., respectively (F). Typically, apoptotic cells (condensed and fragmented nuclei and eosinophilic cytoplasma) are indicated by arrows. Scale bar equals 20 μm.

Figure 6

Histopathological analysis of a small remnant of tumour tissue in an animal with CR 5 months after administration of 120 MBq [¹⁷⁷Lu-DOTA⁰-Tyr³]-octreotate. The tumour residue consisted of a brownish nodule (2 mm in diameter). In the periphery, only a thin rim of fibroblasts was found. The dominant part of the nodule contained crystalline structures surrounded by inflammatory cells including macrophages and giant cells. Scale bar equals 1 mm.