Monique de Visser

Update: Improvement Strategies for Peptide Receptor Scintigraphy and Radionuclide Therapy

Somatostatin receptor-targeting peptides are widely used for the imaging and therapy of neuroendocrine tumors. Peptide-receptor radionuclide therapy (PRRT) in neuroendocrine tumor patients with radiolabeled somatostatin analogs has resulted in symptomatic improvement, prolonged survival, and enhanced quality of life. The side-effects of PRRT are few and mostly mild, certainly when using kidney protective agents. If a more widespread use of PRRT is possible, such therapy might become the therapy of first choice in patients with metastasized or inoperable neuroendocrine gastroenteropancreatic tumors. Yet, much profit can be gained from improving the receptor-targeting strategies available and developing new strategies. This review presents an overview of several options to optimize receptor-targeted imaging and radionuclide therapy. These include the optimization of peptide analogs, increasing the number of receptors on the tumor site, and combining PRRT with other treatment strategies. The development of new peptide analogs with increased receptor-binding affinity and improved stability might lead to a higher accumulation of radioactivity inside tumor cells. Analogs of somatostatin have been widely studied. However, much profit can be gained in improving peptide analogs targeting other tumor-related receptors, including gastrin-releasing peptide (GRP) receptors, neurotensin (NT) receptors, cholecystokinin (CCK) receptors, and glucagon-like peptide-1 (GLP-1) receptors. Several peptide analogs targeting these receptors are well on their way to clinical utilization. The literature shows that it is possible to increase the receptor density on tumor cells by using different methods, which results in higher binding and internalization rates and thus a higher contrast during peptide-receptor scintigraphy. In PRRT treatment, this would enable the administration of higher therapeutic doses to tumors, which might lead to a higher cure rate in patients. Combinations of radionuclide therapy with other treatment modalities, such as chemotherapy or pretreatment with radiosensitizers, might increase the impact of the treatment. Further, the administration of higher dosages of radioactivity to the patient, enabled by combinations of PRRT with strategies reducing the radiation dose to healthy organs, will improve the outcome of tumor treatment. Also, targeting one or several tumor-specific receptors by using combinations of therapeutic agents, as well as by reducing nontarget uptake of radioactivity, will enlarge the therapeutic window of PRRT. Clinical studies will provide more insight in the effects of combining treatment strategies in cancer patients.

Versatile conjugation of octreotide to dendrimers by cycloaddition ("click") chemistry to yield high-affinity multivalent cyclic Peptide dendrimers.

The somatostatin analogue Tyr(3)-octreotide, which has a high binding affinity for the SSTR2 receptor (somatostatin receptor subtype 2) expressed on tumor cells, is used clinically for the diagnosis and treatment of a variety of neuroendocrine tumors and gastrointestinal disorders. There is growing interest in the development of multivalent peptide systems, because they may have enhanced binding affinity compared to monovalent analogues. In this report, we describe the design and synthesis of a series of Tyr(3)-octreotide-containing monomeric, dimeric, and tetrameric dendrimeric conjugates. These multivalent dendrimeric cyclic peptides were obtained using Cu(I)-catalyzed 1,3-dipolar cycloaddition between peptidyl azides and dendrimeric alkynes. Their affinities for the SSTR2 receptor were determined by a competitive binding assay on rat brain sections.

Dynamic and Static Small-Animal SPECT in Rats for Monitoring Renal Function After 177Lu-Labeled Tyr3-Octreotate Radionuclide Therapy

High kidney radiation doses during clinical peptide receptor radionuclide therapy (PRRT) with β-particle–emitting radiolabeled somatostatin analogs will lead to renal failure several months after treatment, urging the coinfusion of the cationic amino acids lysine and arginine to reduce the renal radiation dose. In rat PRRT studies, renal protection by the coadministration of lysine was confirmed by histologic examination of kidney specimens indicating nephrotoxicity. In the current study, we investigated dedicated small-animal SPECT/CT renal imaging in rats to monitor renal function in vivo during follow-up of PRRT, with and without lysine. Methods: The following 3 groups of rats were imaged using a multipinhole SPECT/CT camera: controls (group 1) and rats at more than 90 d after therapy with 460 MBq (15 μg) of 177Lu-DOTA-Tyr3-octreotate without (group 2) or with (group 3) a 400-mg/kg lysine coinjection as kidney protection (n ≥ 6 per group). At 90 and 140 d after therapy, static kidney scintigraphy was performed at 2 h after injection of 25 MBq of 99mTc-dimercaptosuccinic acid (99mTc-DMSA). In addition, dynamic dual-isotope renography was performed using 50 MBq of 111In-diethylenetriaminepentaacetic acid (111In-DTPA) and 50 MBq of 99mTc-mercaptoacetyltriglycine (99mTc-MAG3) at 100–120 d after therapy. Results: 111In-DTPA and 99mTc-MAG3 studies revealed a time–activity pattern comparable to those in patients, with a peak at 2–6 min followed by a decline of renal radioactivity. Reduced 111In-DTPA, 99mTc-MAG3, and 99mTc-DMSA uptake indicated renal damage in group 2, whereas group 3 showed only a decrease of 99mTc-MAG3 peak activity. These results indicating nephrotoxicity in group 2 and renal protection in group 3 correlated with levels of urinary protein and serum creatinine and urea and were confirmed by renal histology. Conclusion: Quantitative dynamic dual-isotope imaging using both 111In-DTPA and 99mTc-MAG3 and static 99mTc-DMSA imaging in rats is feasible using small-animal SPECT, enabling longitudinal monitoring of renal function. 99mTc-MAG3 renography, especially, appears to be a more sensitive marker of tubular function after PRRT than serum chemistry or 99mTc-DMSA scintigraphy.

Androgen-regulated gastrin-releasing peptide receptor expression in androgen-dependent human prostate tumor xenografts.

Human prostate cancer (PC) overexpresses the gastrin‐releasing peptide receptor (GRPR). Radiolabeled GRPR‐targeting analogs of bombesin (BN) have successfully been introduced as potential tracers for visualization and treatment of GRPR‐overexpressing tumors. A previous study showed GRPR‐mediated binding of radiolabeled BN analogs in androgen‐dependent but not in androgen‐independent xenografts representing the more advanced stages of PC. We have further investigated the effect of androgen modulation on GRPR‐expression in three androgen‐dependent human PC‐bearing xenografts: PC295, PC310 and PC82 using the androgen‐independent PC3‐model as a reference. Effects of androgen regulation on GRPR expression were initially studied on tumors obtained from our biorepository of xenograft tissues performing reverse transcriptase polymerase chain reaction (RT‐PCR) and autoradiography (125I‐universal‐BN). A prospective biodistribution study (111In‐MP2653) and subsequent autoradiography (125I‐GRP and 111In‐MP2248) was than performed in castrated and testosterone resupplemented tumor‐bearing mice. For all androgen‐dependent xenografts, tumor uptake and binding decreased drastically after 7 days of castration. Resupplementation of testosterone to castrated animals restored GRPR expression extensively. Similar findings were concluded from the initial autoradiography and RT‐PCR studies. Results from RT‐PCR, for which human specific primers are used, indicate that variations in GRPR expression can be ascribed to mRNA downregulation and not to castration‐induced reduction in the epithelial fraction of the xenograft tumor tissue. In conclusion, expression of human GRPR in androgen‐dependent PC xenografts is reduced by androgen ablation and is reversed by restoring the hormonal status of the animals. This knowledge suggests that hormonal therapy may affect GRPR expression in PC tissue making GRPR‐based imaging and therapy especially suitable for non‐hormonally treated PC patients.

Peptides for Radionuclide Therapy

Somatostatin receptor-targeting peptides are widely being used for imaging and therapy of neuroendocrine tumors. Peptide receptor radionuclide therapy (PRRT) with e.g. 177Lu labeled somatostatin analogues in neuroendocrine tumor patients has resulted in symptomatic improvement, prolonged survival and enhanced quality of life. Yet, much profit can be gained from improving the receptor-targeting strategies available and developing new strategies, e.g. targeting other tumor-specific receptors, such as gastrin-releasing peptide (GRP) receptors and gastrin/cholecystokinin (CCK) receptors, and combining PRRT with other treatment strategies like chemotherapy or co-treatment with radiosensitizers.