Keywords

35.1 Introduction

Somatostatin receptor (SST) scintigraphy for imaging and somatostatin analogues for treatment have been used for the management of patients for more than 20 years. In the last 20 years, important developments have improved the management of patients with neuroendocrine tumours (NETs) or neuroendocrine neoplasias (NENs): (1) introduction of peptide receptor radionuclide therapy (PRRT) with radiolabelled SST agonists such as 90Y- or 177Lu-DOTA-TOC and 177Lu-DOTA-TATE [1], (2) invention of SST PET/CT with radiolabelled SST agonists, such as 68Ga-DOTA-TOC, 68Ga-DOTA-TATE and 68Ga-DOTA-NOC which allows most sensitive staging and restaging of NETs as well as the identification of those patients who will benefit from PRRT (theranostic approach) [2], (3) evaluation of PRRT in a randomized, controlled phase III trial with an intervention arm (177Lu-DOTA-TATE plus somatostatin analogue octreotide LAR), and a control arm (high-dose octreotide LAR) showing the superiority of PRRT in comparison to the treatment with somatostatin analogues [3], (4) discovery that PRRT with α-emitters is likely to perform better than with β-emitters in certain conditions [4], (5) last, but not least, introduction of SST antagonists, which seem to recognize more bindings sites on SST-expressing cancer cells and show favourable pharmacokinetics and better tumour visualization than agonists despite of very poor internalisation rates [5, 6].

Current preclinical and clinical developments of radiolabelled SST antagonists for theranostics (imaging and therapy) and their clinical potential, not only in NETs but also on other tumours are discussed here.

35.2 Part I. Preclinical Development of SST Antagonists for Theranostics

More than 20 years ago, Bass et al. found that the inversion of chirality at position 1 and 2 of the octapeptide (octreotide family) converted an agonist into a potent antagonist [7]. Afterwards, structure activity relationship studies done by Hocart et al. revealed different potent antagonists [8] which were used as lead structures by Jean Rivier (Salk Institute for Biologic Studies, La Jolla, CA), Jean Claude Reubi (University of Bern, Switzerland), and Helmut R. Mäcke (University Hospital Basel, Switzerland) for the collaborative development of SST antagonists for labelling with radiometals [5, 9].

The first radiolabelled SST antagonists were labelled with Indium-111 (111In) via the DOTA (1,4,7,10-tetraazacyclododecan-1,4,7,10-tetraacetic acid) chelator and were based on the somatostatin receptor subtype 2 (SST2)-specific antagonist BASS (p-NO2-Phe-cyclo(D-Cys-Tyr-D-Trp-Lys-Thr-Cys)D-Tyr-NH2) (Table 35.1), developed by Bass et al. [7], and the SST3-specific antagonist SST3-ODN-8, developed by Reubi et al. [10]. Comparison of these new SST selective antagonists (111In-DOTA-BASS and 111In-DOTA-SST3-ODN-8) with highly potent SST agonists (111In-DTPA-TATE which is SST2 specific and 111In-DOTA-NOC which has affinity for SST3, in addition to SST2 and SST5) showed somewhat unexpected results in mice bearing human SST2- and SST3-expressing xenografts:

  1. 1.

    Tumour uptake was more than 1.5-fold higher with the antagonist, despite of the lower receptor affinity [5].

  2. 2.

    Tumour uptake was longer lasting with the antagonist, despite the lack of tumour cell internalisation [5, 11].

Table 35.1 Affinities of metallated somatostatin analogues (IC50 in nM)
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An important finding of these first studies was that radiolabelled SST antagonists recognized a larger number of binding sites in vitro than radiolabelled SST agonists [5].

35.2.1 Second Generation of Radiolabelled SST Antagonists

The same collaborative group (Jean Rivier, Jean Claude Reubi and Helmut R. Mäcke) designed the second generation of SST antagonists with improved SST2 affinities for labelling with the positron emitter Gallium-68 (68Ga) for PET/CT imaging as well as β−-emitters (177Lu and 90Y) for therapy: this included different SST2 specific antagonists such as JR10 (p-NO2-Phe-cyclo-D-Tyr-NH2), JR11 (Cpa-cyclo-D-Tyr-NH2), and LM3 (p-Cl-Phe-cyclo[D-Cys-Tyr-D-Aph(Cbm)-Lys-Thr-Cys]D-Tyr-NH2) [9, 12, 13], in combination with two chelators, namely, DOTA and NODAGA (1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid), (Table 35.1).

Biodistribution and affinity studies with 68Ga-labelled DOTA- and NODAGA-SST2 antagonists indicated that the chelate made the difference as 68Ga-NODAGA conjugates improved the affinity to SST2 and increased the SST2-specific tumour uptake [13, 14]. Furthermore, the Ga(III)-DOTA-SST2 antagonists had a lower affinity for SST2 than the respective Y(III)-DOTA-, Lu(III)-DOTA-, or In(III)-DOTA-SSTR2 antagonists (Table 35.1), [13]. For the understanding of the potential of radiolabelled SST2 antagonists, the comparison with high-affinity SST2 agonists is crucial. For example, 68Ga-DOTA-JR11 and 68Ga-NODAGA-JR11, having a lower affinity for SST2 (~145-fold and ~ six-fold) than 68Ga-DOTA-TATE (Table 35.1), demonstrated in vivo tumour uptake that was 1.3-fold and 1.7-fold higher in a preclinical head-to-head comparison [13].

SST2 antagonists indicated not only superior biodistribution and tumour uptake in combination with 68Ga for PET/CT imaging but also with β−-emitters for a therapeutic approach. For example, a head-to-head comparison of 10 pmol 177Lu-DOTA-JR11 and 10 pmol 177Lu-DOTA-TATE showed significantly higher tumour uptake for 177Lu-DOTA-JR11 (11.7 ± 2.15% injected activity per gram) than for 177Lu-DOTA-TATE (3.66 ± 0.54% injected activity per gram) at 72 h after injection resulting in a 2.6 times higher tumour radiation dose [14]. Importantly, also tumour-to-background dose ratios (kidney, bone marrow, spleen, and liver) were higher with 177Lu-DOTA-JR11 than with 177Lu-DOTA-TATE. The tumour-to-background dose ratio could be further enhanced by increasing the amount of 177Lu-DOTA-JR11 from 10 pmol to 200 pmol (e.g., tumour-to-liver dose ratios were 20.9 with 10 pmol peptide mass and 44.9 with 200 pmol peptide mass) [14]. Comparison of 177Lu-DOTA-JR11 and 177Lu-DOTA-TATE in a mice xenograft study indicated a higher median survival rate (71 vs. 61 day) and a longer delay in tumour growth (26 ± 7 vs. 18 ± 5 day) in 177Lu-DOTA-JR11 treated mice [18]. Similar results are found by Albrecht et al. comparing 177Lu-DOTA-JR11 with 177Lu-DOTA-TOC [19]. Despite the fact that 88 ± 1% of the SST2 antagonist remained on the surface of the tumour cells, 177Lu-DOTA-JR11 showed several time higher tumour uptake and caused at least 60% more DNA double-strand breaks than 177Lu-DOTA-TATE [18]. Head-to-head comparison of 90Y-DOTA-JR11 and 177Lu-DOTA-JR11 revealed a lower therapeutic index of 90Y-DOTA-JR11 with a ~ 20% lower tumour-to-kidney uptake ratio and a > 4 times higher effective dose in treated mice [14]. Furthermore, 111In-DOTA-JR11 cannot be used as a surrogate of 90Y-DOTA-JR11 for imaging and dosimetry studies because of differences in their pharmacokinetics and affinity for SST2 [13, 14].

Based on the affinity profile and preclinical in vivo studies [9, 12, 13] the following radiotracers were advancing into patients:

  1. 1.

    68Ga-NODAGA-JR11 and 68Ga-NODAGA-LM3 for PET/CT imaging.

  2. 2.

    177Lu-DOTA-JR11 for therapy with its theranostic companion 68Ga-DOTA-JR11 and 68Ga-NODAGA-JR11.

  3. 3.

    177Lu-DOTA-LM3 for therapy with its theranostic companion 68Ga-NODAGA-LM3 and and 68Ga-DOTA-LM3.

35.3 Part II. Novel Indications for Theranostics with SST Antagonists

The binding capacity of radiolabelled SST antagonists and agonists were compared in human tissue samples from nine different tumours using in vitro autoradiography with the following SST antagonist/agonist pair: 125I-JR11/125I-Tyr3-octreotide [20] and 177Lu-DOTA-BASS/177Lu-DOTA-TATE [21], (Table 35.2). The SST2 binding affinities (IC50) of the SST antagonist/agonist pair were similar [20, 21]. Importantly, in all cases, the radiolabelled SST antagonist bound to more SST2 sites in all tumours with an uptake that was 3.8 to 21.8 times higher than with the agonist (Table 35.2). Of particular interest is the fact that tumours other than gastroenteropancreatic neuroendocrine tumours (GEP-NETs) and lung NETs have the potential to become targets for radiolabelled SST2 antagonists despite of the relatively low SST2 expression, for example: non-Hodgkin lymphomas, renal cell carcinoma, breast cancer, pheochromocytoma, paraganglioma, medullary thyroid cancer, small cell lung cancer and paraganglioma.

Table 35.2 SST2 binding in different human tumours
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35.4 Part III. Clinical Development of SST Antagonists

Based on these promising in vitro human tumour data as well as in vivo animal data transition into clinic was started with the most promising diagnostic and therapeutic SST2 antagonists.

35.4.1 Studies with Diagnostic SST Antagonists

First clinical evidence that imaging with SST2 antagonists may be superior to agonists was published in 2011 [6]. In this prospective study, 111In-DOTA-BASS total body scintigraphy and SPECT/CT was compared with the U.S. Food and Drug Administration-approved radiotracer 111In-DTPA-octreotide (OctreoScan, Mallinckrodt) and contrast-enhanced CT studies in the same five patients with NETs or thyroid cancer. The affinity profile of 111In-DOTA-BASS and 111In-DTPA-octreotide are in the same range (Table 35.1). A lesion-based analyses revealed a higher tumour detection rate with 111In-DOTA-BASS (25/28 lesions) than with 111In-DTPA-octreotide (17/28 lesions).

Based on affinity studies and preclinical results, the second generation of SST2 antagonist 68Ga-NODAGA-JR11 = 68Ga-OPS202 (Table 35.1) was selected for PET/CT imaging studies. Nicolas et al. performed a single-center, prospective phase I/II study with 12 GEP-NET patients comparing PET/CT with two microdoses of 68Ga-NODAGA-JR11 and one microdose of the potent SSTR2 agonist 68Ga-DOTA-TOC (ClinicalTrials.gov identifier: NCT02162446). The amount and activity of 15/50 μg and 150 MBq 68Ga-NODAGA-JR11 was well tolerated and showed favourable dosimetry results and imaging properties with best tumour contrast between 1 and 2 h after injection [22]. Lesion-based comparison with 68Ga-DOTA-TOC PET/CT showed a significantly higher sensitivity for 68Ga-NODAGA-JR11 PET/CT: 93.7% (95% CI: 85.3–97.6%) vs. 59.2% (95% CI: 36.3–79.1%) [23]. In this study, diagnostic efficacy measures were compared against contrast-enhanced CT or MRI as the standard for comparison.

Several studies were performed with 68Ga-DOTA-JR11 despite its 24 times lower affinity for SST2 compared to 68Ga-NODAGA-JR11 (Table 35.1) [24,25,26]. Zhu et al. compared prospectively 68Ga-DOTA-JR11 and 68Ga-DOTA-TATE PET/CT in the same patients with NETs [25]. As in the study of Nicolas et al. they detected significantly more liver lesions with the SST2 antagonist (552 vs. 365), but at the same time significantly less bone lesions (158 vs. 388), compared to 68Ga-DOTA-TATE. Importantly, 68Ga-DOTA-JR11 showed a lower tumour uptake than the SST2 agonist 68Ga-DOTA-TATE. This is in contrast to the study of Nicolas et al. who prospectively compared 68Ga-NODAGA-JR11 and 68Ga-DOTA-TOC PET/CT in the same patients [23]. Zhu et al. identified two reasons for this finding: (1) 68Ga-DOTA-JR11 has a much lower affinity for SST2 than 68Ga-NODAGA-JR11 (Table 35.1), (2) the study design may cause a bias as 68Ga-DOTA-TATE PET/CT was always performed 24 h ahead of 68Ga-DOTA-JR11 PET/CT. This can cause a saturation/internalisation of SST2 [27]. The different comparator—68Ga-DOTA-TATE instead of 68Ga-DOTA-TOC—is unlikely a confounder explaining the different findings as 68Ga-DOTA-TOC showed higher tumour uptake than 68Ga-DOTA-TATE in a previous study [28].

68Ga-NODAGA-LM3 is another antagonist with similar SST2 affinity as 68Ga-NODAGA-JR11 (Table 35.1). So far, there were only abstracts available with a brief summary of results from two retrospective compassionate use studies with 68Ga-NODAGA-LM3 PET/CT. The first study showed in 40 patients with GEP-NET, lung NET, paraganglioma/pheochromocytoma etc. that PET/CT imaging with 68Ga-NODAGA-LM3 is feasible [29]. The other study compared 68Ga-NODAGA-LM3 and 68Ga-DOTA-TOC PET/CT in ten paraganglioma patients. 68Ga-NODAGA-LM3 PET/CT detected many more lesions (243 vs. 177) including many more bone lesions (190 vs. 143) than 68Ga-DOTA-TOC PET/CT [30].

35.4.2 Studies with Therapeutic SST Antagonists

Based on affinity studies and preclinical results, the second generation of SST2 antagonist 177Lu-DOTA-JR11 = 177Lu-OPS201 (Table 35.1) was selected for a therapeutic first-in-human study: In a single-centre, prospective proof-of-principle study (phase 0 study), tumour and organ doses of 177Lu-DOTA-JR11 and 177Lu-DOTA-TATE were compared in the same four patients with advanced, metastatic neuroendocrine neoplasia (NEN), grade 1–3 [31]. The most relevant findings were a 3.5-fold higher median tumour dose as well as >two-fold higher tumour-to-kidney dose ratios with 177Lu-DOTA-JR11 compared to 177Lu-DOTA-TATE, tumour doses of up to 487 Gy and moderate adverse events with one grade 3 thrombocytopenia after treatment with three cycles (total 15.2 GBq) of 177Lu-DOTA-JR11. Reidy-Lagunes et al., however, described grade 4 hematotoxicity (leukopenia, neutropenia, and thrombocytopenia) in four of the first seven patients with NETs treated with two cycles of 177Lu-DOTA-JR11 (total activity between 10.5 and 14.7 GBq) [32]. Hence, their single-centre phase I study was suspended, and the protocol modified to limit the cumulative absorbed bone marrow dose. The most important results of the whole study are summarized in Table 35.3. 177Lu-DOTA-JR11 (177Lu-OPS201) is currently evaluated in a phase I/II multicentre study (ClinicalTrials.gov identifier: NCT02162446) and its “sister” compound 177Lu-DOTA-LM3 is evaluated in a single-centre compassionate use study. So far, there were only abstracts available with a brief summary of results from both studies [33, 34]. Table 35.3 shows the most important findings of those studies.

Table 35.3 Summary of study results with therapeutic SST antagonists
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35.5 Part IV. Current and Future Developments

The high potential and promising results of diagnostic and therapeutic radiolabelled SST antagonists have attracted several research groups to further evaluate radiolabelled SST antagonists. Table 35.4 shows an overview of such studies that are listed within ClinicalTrials.gov. The results of these studies are expected to be published in the near future.

Table 35.4 Overview about current registered studies
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The antagonist approach has huge potential to offer new and better theranostic procedures for patients. Here is an overview about possible future developments to achieve this ambition:

  1. 1.

    There are other tumours than GEP-NETs that show high potential for theranostic applications with SST antagonists, for example, non-Hodgkin lymphoma, renal cell carcinoma, breast cancer, pheochromocytoma, paraganglioma, medullary thyroid cancer, small cell lung cancer, lung NET, and other neoplasms with SST2 expression, including tumours with low levels of SST2 expression.

  2. 2.

    Optimizing the SST antagonist approach which includes the reduction of bone marrow toxicity by using alternative radionuclides, for example, α-emitters and β−-emitters with suitable characteristics such as long half-lives, short range, etc.

  3. 3.

    Using other receptor systems for the antagonist approach, for example, gastrin-releasing peptide receptor and cholecystokinin receptor subtype 2.

Last, but not least randomized phase II/III studies evaluating radiolabelled NODAGA-JR11/DOTA-JR11, radiolabelled NODAGA-LM3/DOTA-LM3 or other promising radiolabelled SST antagonists are needed in larger-scale multicentre trials as a theranostic approach in patients with GEP-NETs or other tumours with SST2 expression.

35.6 Conclusion

Until recently it was thought that internalisation of the radiotracer was mandatory for diagnostic and therapeutic SST targeting. Ginj et al. proposed 15 years ago that radiolabelled SST antagonists are superior to SST agonists despite the lack of internalisation [5]. Recently it has been shown that 68Ga-NODAGA-JR11 and 68Ga-NODAGA-LM3 PET/CT revealed the best clinical results among all tested SST antagonists and are clearly superior compared to PET/CT with the potent SST2 agonist 68Ga-DOTA-TOC. PRRT with 177Lu-DOTA-JR11 and 177Lu-DOTA-LM3 is very effective with a high objective response rate (30–45%) and an excellent 1-year PFS of 90% despite of using treatment protocols with low activities (< 15 GBq) given in one to three cycles. The dose limiting organ is the bone marrow, at least for 177Lu-DOTA-JR11. These results warrants larger-scale randomized phase II/III trials in patients with GEP-NETs and tumours that have not yet been in the focus for SST targeting. Current evidence from preclinical work, binding capacity studies with different human tumour samples and clinical studies support the shift towards SST antagonists.