Tumor uptake of 68Ga-DOTA-Tyr3-octreotate: animal PET studies of tumor flow and acute somatostatin receptor modulation in the CA20948 rat model
Introduction
Most neuroendocrine tumors express high density of somatostatin receptors (sstr) at the cell surface. Five human somatostatin receptors have been identified and cloned (hsstr1-5). Neuroendocrine tumors express mainly sstr2 and to a lower extent sstr5; hence, agonists have been developed to specifically target these receptors [1]. The presence of these receptors has been exploited for the diagnosis of such tumors, by using somatostatin (sst) analogs, labeled with gamma emitters such as 111In [2], [3], or with positron emitters (86Y, 68Ga) [4], [5], [6], [7]. sst Analogs can also be applied for peptide receptor radionuclide therapy (PRRT) by using high activities of peptides labeled with either 111In for its Auger electron emission [8], [9], [10], [11], or beta-emitting radionuclides such as 177Lu and 90Y [12], [13].
The dose delivered to the tumor is determined by the injected activity, which in turn, is limited by the maximal dose that critical organs (i.e., the kidneys and bone marrow) can tolerate. Several approaches were pursued to enhance the dose to tumors by increasing injected activities while avoiding kidney failure [14] and acute or chronic bone marrow toxicity. In particular, efforts were dedicated to reduce the renal uptake of labeled peptides using amino acids [15] or other substances interfering with the re-uptake by the renal proximal tubular cells [16]. The assessment of individual patient dosimetry has also shown promising to optimize the efficacy/toxicity ratio [17].
As of today, limited attention was paid to enhance the actual tumor uptake of radiolabeled analogs which would result in increased tumor doses, without increasing toxicity. In particular, the blood delivery of radiolabeled analogs and the mechanisms of sstr expression on tumors in vivo and of potential ways to modulate them have not been extensively studied. To the best of our knowledge, tumor blood flow has not been studied in vivo in experimental and human neuroendocrine tumors. Modulation of blood flow, if feasible, could be a way to improve gross tumor uptake.
On the other hand, two studies showed that in vitro exposure of neuroendocrine tumor cells to cold somatostatin analogs increases the number of receptors at the cell surface. Hukovic et al. demonstrated this effect in vitro for human (h) sstr within 14 hours. However, the observed up-regulation was shown to vary according to the receptor subtype, hsstr1 showing higher up-regulation, whereas up-regulation of hsstr2 and hsstr4 was limited [18]. Froidevaux et al. studied the expression of sstr2 both in vitro and in vivo on AR42J tumor cells implanted in mice after acute or chronic exposure to octreotide [19]. Compared to a control group, animals treated with continuous release of octreotide over 7 days showed an increased expression of sstr2.
The purposes of this PET study were first, to evaluate whether tumor blood flow, measured using the short half-lived H215O in CA20968, as one of the parameters involved in the uptake of radiolabeled sst analogs, could be studied quantitatively using animal PET and second, to analyze the changes in tumor uptake of a [68Ga]-labeled somatostatin analog, DOTA-Tyr3-octreotate, following treatment with the cold analog octreotide. The studies aimed to take advantage of animal PET scanning combined with short-lived radiotracers such as 68Ga and to exploit it to follow the same animal over time to assess the potential impact of pharmacological intervention.
Section snippets
Animal and tumor model
Male Lewis rats (7 weeks old) were injected subcutaneously with 1 ml of tumor cell suspension (<108 cells) prepared with 1 g of crude CA20948 tumor tissue in 10 ml DMEM. Rats were purchased from Charles River (Margate, Kent, UK). Tumors were allowed to grow for 15–20 days after inoculation before imaging was performed. Animals were housed two or three per cage and fed ad libitum. All imaging procedures were performed under continuous isoflurane anesthesia (induction 3% - maintenance 1.5%;
Blood flow
Reproducibility of H215O myocardium flow measurements was high with K1 values of 5.70, 5.23 and 5.58 ml/min per gram of tissue.
In the kidney, respective K1 values were 5.59, 5.34 and 5.51. These values indicated that blood flow measurements are reliable in highly perfused organs.
The blood flow measured using the microsphere method was very low as indicated by the minimal 57Co microsphere activity in the tumors, resulting in tumor-to-myocardium activity ratios that ranged between 0.01 and 0.02.
Discussion
These studies show the potential and limitations of PET to evaluate factors that could be essential for the uptake of sst analogs by neuroendocrine tumors in vivo. They show that the noninvasive assessment of pharmacological interventions in small animals using an animal PET scanner is feasible. No evidence of up-regulation of sstr2 receptors by a single iv injection of cold octreotide was demonstrated. On the other hand, the uptake of the radiolabeled analog following injection of cold
Acknowledgments
The authors address special thanks to Bert Bernard, Flavio Forrer, Daniel Labar and Pascal Carlier for excellent technical support and fruitful discussions.
References (37)
- et al.
Phase I study of peptide receptor radionuclide therapy with [In-DTPA]octreotide: the Rotterdam experience
Semin Nucl Med
(2002) - et al.
Somatostatin receptor scintigraphy during treatment with lanreotide in patients with neuroendocrine tumors
Nucl Med Biol
(1999) - et al.
Affinity profiles for human somatostatin receptor subtypes SST1-SST5 of somatostatin radiotracers selected for scintigraphic and radiotherapeutic use
Eur J Nucl Med
(2000) - et al.
Somatostatin receptor scintigraphy with [111In-DTPA-D-Phe1]- and [123I-Tyr3]-octreotide: the Rotterdam experience with more than 1000 patients
Eur J Nucl Med
(1993) - et al.
Somatostatin-receptor imaging in the localization of endocrine tumors
N Engl J Med
(1990) - et al.
Radiolabelling DOTA-peptides with 68Ga
Eur J Nucl Med Mol Imaging
(2005) - et al.
Biokinetics and imaging with the somatostatin receptor PET radioligand (68)Ga-DOTATOC: preliminary data
Eur J Nucl Med
(2001) - et al.
86Y-DOTA0)-D-Phe1-Tyr3-octreotide (SMT487)–a phase 1 clinical study: pharmacokinetics, biodistribution and renal protective effect of different regimens of amino acid co-infusion
Eur J Nucl Med Mol Imaging
(2003) - et al.
(68)Ga-labeled peptides in tumor imaging
J Nucl Med
(2005) - et al.
Receptor targeting for tumor localisation and therapy with radiopeptides
Curr Med Chem
(2000)
Radiolabelled somatostatin analogue(s) for peptide receptor scintigraphy and radionuclide therapy
Ann Oncol
Peptide receptor imaging and therapy
J Nucl Med
[177Lu-DOTAOTyr3]octreotate: comparison with [111In-DTPAo]octreotide in patients
Eur J Nucl Med
Yttrium-90 DOTATOC: first clinical results
Eur J Nucl Med
Long-term follow-up of renal function after peptide receptor radiation therapy with (90)Y-DOTA(0),Tyr(3)-octreotide and (177)Lu-DOTA(0), Tyr(3)-octreotate
J Nucl Med
Metabolic effects of amino acid solutions infused for renal protection during therapy with radiolabelled somatostatin analogues
Nephrol Dial Transplant
Renal uptake of radiolabeled octreotide in human subjects is efficiently inhibited by succinylated gelatin
J Nucl Med
Therapy using labelled somatostatin analogues: comparison of the absorbed doses with 111In-DTPA-D-Phe1-octreotide and yttrium-labelled DOTA-D-Phe1-Tyr3-octreotide
Nucl Med Commun
Cited by (10)
Preclinical evaluation of a dual sstr2 and integrin α<inf>v</inf>β<inf>3</inf>-targeted heterodimer [<sup>68</sup>Ga]-NOTA-3PEG<inf>4</inf>-TATE-RGD
2019, Bioorganic and Medicinal ChemistryCitation Excerpt :Most of NET express high level of somatostatin receptors (sstr) 1–5 at the cell membrane. To be exact, NET express mainly sstr2 and a lower extent sstr5.5,6 Based on that theory, precision diagnosis of SCLC or NSCLC using 68Gallium (68Ga) labelled TATE or RGD showed good results.7–9.
Hepatocellular carcinoma and gastroenteropancreatic neuroendocrine tumors: Potential role of other positron emission tomography radiotracers
2012, Seminars in Nuclear MedicineCitation Excerpt :A potential confounding factor for PET tracer avidity to the somatostatin receptor may be competition with therapeutic unlabeled somatostatin analogs, such as octreotide or drug-induced internalization of the somatostatin subtype 2 receptors. Animal model studies have shown that acute exposure to unlabeled octreotide results in rapid recycling and resynthesis of somatostatin receptors.48 Haug et al49 compared 68Ga-DOTA-TATE uptake in 2 groups of patients, with 1 group pretreated with long-acting octreotide.
The Use of the Macrocyclic Chelator DOTA in Radiochemical Separations
2020, European Journal of Inorganic ChemistrySafety and efficacy of <sup>68</sup>Ga-DOTATATE PET/CT for diagnosis, staging, and treatment management of neuroendocrine tumors
2016, Journal of Nuclear MedicineProspective of <sup>68</sup>Ga-Radiopharmaceutical development
2014, Theranostics