Twins in spirit: DOTATATE and high-affinity DOTATATE

A physiologically based pharmacokinetic model for [68Ga]Ga-(HA-)DOTATATE to predict whole-body distribution and tumor sink effects in GEP-NET patients

BACKGROUND

Little is known about parameters that have a relevant impact on (dis)similarities in biodistribution between various ⁶⁸Ga-labeled somatostatin analogues. Additionally, the effect of tumor burden on organ uptake remains unclear. Therefore, the aim of this study was to describe and compare organ and tumor distribution of [⁶⁸Ga]Ga-DOTATATE and [⁶⁸Ga]Ga-HA-DOTATATE using a physiologically based pharmacokinetic (PBPK) model and to identify factors that might cause biodistribution and tumor uptake differences between both peptides. In addition, the effect of tumor burden on peptide biodistribution in gastroenteropancreatic (GEP) neuroendocrine tumor (NET) patients was assessed.

METHODS

A PBPK model was developed for [⁶⁸Ga]Ga-(HA-)DOTATATE in GEP-NET patients. Three tumor compartments were added, representing primary tumor, liver metastases and other metastases. Furthermore, reactions describing receptor binding, internalization and recycling, renal clearance and intracellular degradation were added to the model. Scan data from GEP-NET patients were used for evaluation of model predictions. Simulations with increasing tumor volumes were performed to assess the tumor sink effect.

RESULTS

Data of 39 and 59 patients receiving [⁶⁸Ga]Ga-DOTATATE and [⁶⁸Ga]Ga-HA-DOTATATE, respectively, were included. Evaluations showed that the model adequately described image-based patient data and that different receptor affinities caused organ uptake dissimilarities between both peptides. Sensitivity analysis indicated that tumor blood flow and blood volume impacted tumor distribution most. Tumor sink predictions showed a decrease in spleen uptake with increasing tumor volume, which seemed clinically relevant for patients with total tumor volumes higher than ~ 550 mL.

CONCLUSION

The developed PBPK model adequately predicted tumor and organ uptake for this GEP-NET population. Relevant organ uptake differences between [⁶⁸Ga]Ga-DOTATATE and [⁶⁸Ga]Ga-HA-DOTATATE were caused by different affinity profiles, while tumor uptake was mainly affected by tumor blood flow and blood volume. Furthermore, tumor sink predictions showed that for the majority of patients a tumor sink effect is not expected to be clinically relevant.

Twins in spirit part IV - [177Lu] high affinity DOTATATE. A promising new tracer for peptide receptor radiotherapy?

AIM

Besides the use of somatostatin analogues, small molecules like sunitinib and everolimus as well as conventional chemotherapy, peptide receptor radiotherapy (PRRT) using radiolabelled somatostatin analogues has gained an important role in the treatment of inoperable, metastasized neuroendocrine tumours (NET). There are various radiotracers in use. Based on our experience with the PET tracer [⁶⁸Ga]DOTA-3-iodo-Tyr³-octreotate ([⁶⁸Ga]HA-DOTATATE), a DOTATATE derivative with an increased binding affinity to hsst5, the current retrospective analysis is exploring the therapeutic potential of [¹⁷⁷Lu]HA-DOTATATE.

METHODS

Eighteen patients with metastatic NET (G1/G2) were treated using [¹⁷⁷Lu]DOTATATE and/or [¹⁷⁷Lu]HA-DOTATATE, and dosimetric results of both tracers were compared.

RESULTS

Using [¹⁷⁷Lu]HA-DOTATATE, a mean tumour dose of 5.34 Gy/GBq (median 2.53 Gy/GBq; range 0.89-33.3 Gy/GBq) was achieved, while [¹⁷⁷Lu]DOTATATE delivered a tumour dose of 5.53 Gy/GBq (median 2.70 Gy/GBq; range 0.44-15.3 Gy/GBq). Organ doses for [¹⁷⁷Lu]HA-DOTATATE vs. [¹⁷⁷Lu]DOTATATE were as follows: kidney 2.31 ± 0.85 vs. 2.03 ± 0.96 Gy/GBq, liver 1.06 ± 0.79 vs. 1.67 ± 1.73 Gy/GBq, spleen 3.89 ± 4.04 vs. 4.50 ± 3.69 Gy/GBq and whole body 0.16 ± 0.10 Gy/GBq vs. 0.15 ± 0.08 Gy/GBq. Tumour-to-kidney dose ratio was slightly higher for [¹⁷⁷Lu]DOTATATE (2.4 ± 5.6) compared to [¹⁷⁷Lu]HA-DOTATATE (1.5 ± 3.6).

CONCLUSION

Both tracers showed marked inter-patient variation in their dosimetry, and no significant differences in dosimetry of [¹⁷⁷Lu]HA-DOTATATE and [¹⁷⁷Lu]DOTATATE were observed when taking all patients into account. Thus, [¹⁷⁷Lu]HA-DOTATATE appears viable for PRRT, although it was marginally inferior regarding kidney dose and tumour-to-kidney dose ratio compared to the established [¹⁷⁷Lu]DOTATATE.

Twins in spirit - episode I: comparative preclinical evaluation of [(68)Ga]DOTATATE and [(68)Ga]HA-DOTATATE

Background

Recently, an intra-patient comparison demonstrated that the somatostatin (sst) ligand [⁶⁸Ga]HA-DOTATATE ([⁶⁸Ga]DOTA-3-iodo-Tyr³-octreotate) provides PET images comparable to or superior to those obtained with [⁶⁸Ga]DOTATATE. To provide a comprehensive basis for nevertheless observed slight differences in tracer biodistribution and dosimetry, the characteristics of [⁶⁸Ga]HA-DOTATATE were investigated in a detailed preclinical study.

Methods

Affinities of ^natGa-HA-DOTATATE and ^natGa-DOTATATE to sst_1–5 were determined using membrane preparations and [¹²⁵I]SST-28 as radioligand. Internalization into AR42J cells was studied in dual-tracer studies with [¹²⁵I]TOC as internal reference. Biodistribution was investigated using AR42J tumor-bearing CD1 mice, and specificity of tracer uptake was confirmed in competition studies by coinjection of 0.8 mg TOC/kg.

Results

Sst₂ affinities (IC₅₀) of [^natGa]HA-DOTATATE (1.4 ± 0.8 nM, logP: −3.16) and [^natGa]DOTATATE (1.2 ± 0.6 nM, logP: −3.69) were nearly identical. Both compounds displayed IC₅₀ > 1 μM for sst_1,3,4, while sst₅ affinity was markedly increased for ^natGa-HA-DOTATATE (102 ± 65 nM vs >1 μM for ^natGa-DOTATATE). [^natLu]HA-DOTATATE and [^natLu]DOTATATE showed slightly lower, identical sst₂ affinities (2.0 ± 1.6 and 2.0 ± 0.8 nM, respectively) and sst₃ affinities of 93 ± 1 and 162 ± 16 nM. Internalization of [⁶⁸Ga]HA-DOTATATE was tenfold higher than that of [¹²⁵I]TOC but only sixfold higher for [⁶⁸Ga]DOTATATE and [¹⁷⁷Lu]HA-DOTATATE. While [⁶⁸Ga]HA-DOTATATE and [⁶⁸Ga]DOTATATE had shown similar target- and non-target uptake in patients, biodistribution studies in mice at 1 h post injection (n = 5) revealed slightly increased non-specific uptake of [⁶⁸Ga]HA-DOTATATE in the blood, liver, and intestines (0.7 ± 0.3, 1.0 ± 0.2, and 4.0 ± 0.7 %iD/g vs 0.3 ± 0.1, 0.5 ± 0.1, and 2.7 ± 0.8 %iD/g for [⁶⁸Ga]DOTATATE). However, sst-mediated accumulation of [⁶⁸Ga]HA-DOTATATE in the pancreas, adrenals, and tumor was significantly enhanced (36.6 ± 4.3, 10.8 ± 3.2, and 33.6 ± 10.9 %iD/g vs 26.1 ± 5.0, 5.1 ± 1.4, and 24.1 ± 4.9 %iD/g, respectively). Consequently, tumor/background ratios for [⁶⁸Ga]HA-DOTATATE in the AR42J model are comparable or slightly increased compared to [⁶⁸Ga]DOTATATE.

Conclusions

The present preclinical data fully confirm the general biodistribution pattern and excellent in vivo sst-targeting characteristics previously observed for [⁶⁸Ga]HA-DOTATATE in patients. The effect of slightly enhanced lipophilicity on background accumulation and normal organ dose is compensated by the high uptake of [⁶⁸Ga]HA-DOTATATE in tumor. Thus, [⁶⁸Ga]HA-DOTATATE represents a fully adequate, freely available substitute for [⁶⁸Ga]DOTATATE and, given the superb sst-targeting characteristics of [¹⁷⁷Lu]HA-DOTATATE in vitro, potential applicability for sst-targeted PRRT.

Dosimetric measurements of (68)Ga-high affinity DOTATATE: twins in spirit - part III

PURPOSE

68Ga-labelled compounds are increasingly used for somatostatin-receptor scintigraphy because of their favourable biokinetic properties, a higher tumour-to-background contrast and higher diagnostic accuracy compared to the gamma-emitting tracer 111In-DTPA-octreotide. Recently, we have introduced the new tracer 68Ga-DOTA-3-iodo-Tyr3-Thr8-octreotide (68Ga-HA-DOTATATE). The present study demonstrates the biodistribution and radiation dosimetry of this tracer in humans.

PATIENTS, METHODS

Seven men were enrolled in this analysis. Every patient underwent a 20 min dynamic PET scan after intravenous injection of about 114 ± 9 MBq of 68Ga-HA-DOTATATE. This was followed by two whole-body scans at 30 min p. i. and 120 min p. i. Blood radioactivity concentration was determined non-invasively from a ROI drawn over the aorta. Urine was collected until the time of the last scan. Liver, spleen, kidneys and urinary bladder wall were included in the dosimetric estimation that was carried out with the software package OLINDA 1.0.

RESULTS

Physiological 68Ga-HA-DOTATATE uptake was observed in the pituitary gland, thyroid, salivary glands, liver, spleen, kidneys, urinary bladder, adrenals and intestine. Organs with the highest absorbed dose were spleen (0.26 ± 0.11 mSv/MBq), kidneys (0.14 ± 0.03 mSv/MBq) and liver (0.12 ± 0.02 mSv/MBq).The estimated effective dose was 0.024 ± 0.001 mSv/MBq.

CONCLUSION

Our study demonstrates biokinetics and radiation exposure of the 68Ga-labelled tracer HA-DOTATATE to be comparable to other 68Ga-labelled SSR analogues in clinical use.

Twins in spirit part II: DOTATATE and high-affinity DOTATATE--the clinical experience

PURPOSE

Over recent decades interest in diagnosis and treatment of neuroendocrine tumours (NET) has steadily grown. The basis for diagnosis and therapy of NET with radiolabelled somatostatin (hsst) analogues is the variable overexpression of hsst receptors (hsst1-5 receptors). We hypothesized that radiometal derivatives of DOTA-iodo-Tyr(3)-octreotide analogues might be excellent candidates for somatostatin receptor imaging. We therefore explored the diagnostic potential of (68)Ga-DOTA-iodo-Tyr(3)-octreotate [(68)Ga-DOTA,3-iodo-Tyr(3),Thr(8)]octreotide ((68)Ga-HA-DOTATATE; HA, high-affinity) compared to the established (68)Ga-DOTA-Tyr(3)-octreotate ((68)Ga-DOTATATE) in vivo.

METHODS

The study included 23 patients with known somatostatin receptor-positive metastases from NETs, thyroid cancer or glomus tumours who were investigated with both (68)Ga-HA-DOTATATE and (68)Ga-DOTATATE. A patient-based and a lesion-based comparative analysis was carried out of normal tissue distribution and lesion detectability in a qualitative and a semiquantitative manner.

RESULTS

(68)Ga-HA-DOTATATE and (68)Ga-DOTATATE showed comparable uptake in the liver (SUVmean 8.9 ± 2.2 vs. 9.3 ± 2.5, n.s.), renal cortex (SUVmean 13.3 ± 3.9 vs. 14.5 ± 3.7, n.s.) and spleen (SUVmean 24.0 ± 6.7 vs. 22.9 ± 7.3, n.s.). A somewhat higher pituitary uptake was found with (68)Ga-HA-DOTATATE (SUVmean 6.3 ± 1.8 vs. 5.4 ± 2.1, p < 0.05). On a lesion-by-lesion basis a total of 344 lesions were detected. (68)Ga-HA-DOTATATE demonstrated 328 lesions (95.3% of total lesions seen), and (68)Ga-DOTATATE demonstrated 332 lesions (96.4%). The mean SUVmax of all lesions was not significantly different between (68)Ga-HA-DOTATATE and (68)Ga-DOTATATE (17.8 ± 11.4 vs. 16.7 ± 10.7, n.s.).

CONCLUSION

Our analysis demonstrated very good concordance between (68)Ga-HA-DOTATATE and (68)Ga-DOTATATE PET data. As the availability and use of (68)Ga-HA-DOTATATE is not governed by patent restrictions it may be an attractive alternative to other (68)Ga-labelled hsst analogues.