One decade of ‘Bench-to-Bedside’ peptide receptor radionuclide therapy with indigenous [¹⁷⁷Lu]Lu-DOTATATE obtained through ‘Direct’ neutron activation route: lessons learnt including practice evolution in an Indian setting

Abstract

The present treatise chronicles one decade of experience pertaining to clinical PRRT services in a large-volume tertiary cancer care centre in India delivering over 4,000 therapies, an exemplar of successful PRRT programme employing indigenous ¹⁷⁷Lutetium production and resources. For the purpose of systematic discussion, we have sub-divided the communication into 3 specific parts: (a) Radiopharmaceutical aspects that describes ¹⁷⁷Lutetium production through ‘Direct’ Neutron Activation Route and the subsequent radiolabeling procedures, (b) The specific clinical nuances and finer learning points (apart from the routine standard procedure) based upon clinical experience and how it has undergone practice evolution in our setting and (c) Dosimetry results with this indigenous product and radiation safety/health physics aspects involved in PRRT services. Initiated in 2010 at our centre, the PRRT programme is a perfect example of affordable quality health care delivery, with indigenous production of the radionuclide (¹⁷⁷Lu) in the reactor and subsequent radiolabeling of the radiopharmaceutical ([¹⁷⁷Lu]Lu-DOTATATE) at the hospital radiopharmacy unit of the centre, which enabled catering to the needs of a large number of patients of progressive, metastatic and advanced Neuroendocrine Neoplasms (NENs) and related malignancies.

Introduction

The somatostatin receptor (SSTR)-targeted theranostics integrates [^99mTc]Tc/[¹¹¹In]In/[⁶⁸Ga]Ga-labelled diagnostic SPECT or PET imaging and [¹⁷⁷Lu]Lu/[⁹⁰Y]Y-labelled therapeutic radiopharmaceuticals for ‘Peptide Receptor Radionuclide Therapy’ (PRRT), that has seen rapidly increasing applications for metastatic or advanced and progressive neuroendocrine neoplasms (NENs), and gratifying cumulative clinical experience for disease stabilization in majority and partial response in a sizeable fraction of patients. The somatostatin receptor subtype 2 (SSTR₂), a member of the G-protein coupled somatostatin receptor family (SSTR 1-5) is the principal target for PRRT, in view of its predominance in the NENs. The therapy is accomplished through intravenously administered unsealed radiopharmaceutical, the most common across the world being [¹⁷⁷Lu]Lu-DOTATATE (DOTATATE = DOTA⁰-(Tyr³)-octreotate) [¹⁷⁷Lu: E_β(max) = 0.497 MeV; maximum tissue range ~2.5 mm], while [⁹⁰Y]Y-DOTATATE or DOTATOC is the other beta emitting therapeutic alternative, in which the harder beta energy [⁹⁰Y: E_β(max) = 2.28 MeV] and longer range of ⁹⁰Y (maximum tissue range ~11 mm) makes it more suitable for larger and heterogeneous tumors (albeit also accounting for its more nephrotoxicity), while [¹⁷⁷Lu]Lu-DOTATATE is more efficacious for smaller tumors.

Radiopharmaceutical aspects: the ¹⁷⁷Lutetium revolution

Production of ¹⁷⁷Lu suitable for formulation of [¹⁷⁷Lu]Lu-DOTA⁰-Tyr³-octreotate ([¹⁷⁷Lu]Lu-DOTATATE) for PRRT

Two alternative routes are available for production of ¹⁷⁷Lu with adequate specific activity required for its utility in PRRT, as shown in Figure 1. The first is the “direct” route which involves the thermal neutron activation of highly enriched (in ¹⁷⁶Lu) lutetium target [¹⁷⁶Lu(n, γ)¹⁷⁷Lu] in research reactors with medium to high thermal neutron flux [1-7]. The second route or the “indirect” route is based on neutron irradiation of highly enriched (in ¹⁷⁶Yb) ytterbium targets leading to the formation of no-carrier-added (NCA) ¹⁷⁷Lu from the β- decay of the short-lived activation product ¹⁷⁷Yb (T_1/2 = 1.9 h) [3,8-10]. The post-irradition radiochemical processing in the former case involves simple dissolution of the irradiated target, whereas the latter route involves elaborate procedure to separate ¹⁷⁷Lu from ytterbium targets as well as its radionuclides. The neutron activation products of natural lutetium and ytterbium targets along with the nuclear decay characteristics of the product radionuclides are given in Table 1.

Figure 1.

Alternate routes for production of ¹⁷⁷Lu in Nuclear Reactor.

Table 1.

Neutron activation products of natural lutetium and ytterbium targets along with the nuclear decay characteristics of the product radionuclides

Target element	Target isotope	% Abundance	σ (barns)	Activation product	Mode of decay	T1/2	Decay product
Lu	175Lu	97.41	16.7	176mLu	β-, γ	3.66 h	176Hf
			6.6	176Lu	β-, γ	4×1010 y	176Hf
	176Lu	2.59	2.8	177mLu	β-, γ & IT	160.4 d	177Hf (78.6%)
							177Lu (21.4%)
			2090	177Lu	β-, γ	6.65 d	177Hf
Yb	168Yb	0.13	2300	169Yb	EC	32.02 d	169Tm
	170Yb	3.04	9.9	171Yb	Stable
	171Yb	14.28	58.3	172Yb	Stable
	172Yb	21.83	1.3	173Yb	Stable
	173Yb	16.13	15.5	174Yb	Stable
	174Yb	31.83	63	175Yb	β-, γ	4.18 d	175Lu
	176Yb	12.76	2.85	177Yb	β-, γ	1.9 h	177Lu

The indirect route of production offers two distinct advantages over the direct route, (i) it provides NCA ¹⁷⁷Lu (theoretical specific activity 4.1 TBq/mg, 110.9 Ci/mg), which in turn leads to higher specific activity of the radiolabeled peptide (ii) ¹⁷⁷Lu produced from indirect route is practically free from any radionuclide contaminant, provided a robust radiochemical separation strategy is adapted to separate ¹⁷⁷Lu from radionuclides of ytterbium. Different approaches have been successfully utilized in the separation of clinical grade NCA ¹⁷⁷Lu from bulk quantity of neutron irradiated ytterbium target [8-11]. Although, NCA ¹⁷⁷Lu is available from commercial sources, the absolute need of a complex radiochemical separation procedure to isolate ¹⁷⁷Lu of requisite purity is challenging and expensive. It can be shown by theoretical calculation that irradiation of 1 mg of 99% enriched (in ¹⁷⁶Yb) Yb₂O₃ target at a thermal neutron flux of 5×10¹⁴ n/cm².s will produce only ~3.2 GBq (~87 mCi) of ¹⁷⁷Lu.

In contrast to any other medically useful radionuclides, direct (n, γ) route can be utilized for large-scale production of ¹⁷⁷Lu in nuclear reactors, having medium to high thermal neutron flux (1.0×10¹⁴ n.cm^-2.s^-1 or higher), using enriched lutetium target (80% or more in ¹⁷⁶Lu), with specific activity adequate for preparing receptor-specific therapeutic radiopharmaceuticals. This is possible due to the following two reasons, (i) ¹⁷⁶Lu has very high thermal neutron capture cross section (σ = 2090 b, I₀ = 1087 b) for formation of ¹⁷⁷Lu and also that (ii) the neutron capture cross section of ¹⁷⁶Lu does not follow 1/v law and there is a strong resonance very close to the thermal region [12]. Consequently, it is possible to produce ¹⁷⁷Lu of specific activity of more than 20 Ci/mg (740 GBq/mg) in a medium flux reactor. The most significant advantage of direct route over the indirect route is the simplicity of post-irradiation chemical treatment procedure, which makes it much less technologically demanding as well as cost-effective compared to the other route. Moreover, there is absolutely no impediment towards scaling up the production as per its clinical requirement. On the contrary, the major concern in the large-scale clinical utilization of ¹⁷⁷Lu produced via the direct route, is the presence of the co-produced ^177mLu, with a long T_1/2 of 160.4 days (Table 1), which creates problem in the disposal of radioactive wastes arising from the treatment of large number of patients beside the additional radiation dose burden to the patients. A careful optimization of the duration of irradiation depending on the available thermal neutron flux of the reactor is essential in the direct route to obtain ¹⁷⁷Lu in the highest achievable specific activity while keeping the contamination from ^177mLu to a minimum [1,3,5,8,13].

In India, the clinical grade ¹⁷⁷Lu used in PRRT is indigenously produced following the direct route, by irradiation of enriched lutetium oxide target (~82% enriched in ¹⁷⁶Lu) at a thermal neutron flux of 1.6×10¹⁴ n.cm^-2.s^-1 at Dhruva research reactor, Bhabha Atomic Research Centre (BARC), Mumbai. The irradiated target is dissolved in 0.01 M ultrapure HCl solution in an aseptic environment to obtain sterile, pyrogen free, [¹⁷⁷Lu]LuCl₃ solution as the radiopharmaceutical precursor.

The theoretical yield of ¹⁷⁷Lu ‘A’ (in Bq) produced via ¹⁷⁶Lu(n, γ)¹⁷⁷Lu at the end of irradiation is calculated using the formula (Equation 1):

Where, where, N₀ = number of ¹⁷⁶Lu atoms used as target (at t = 0), λ = decay constant of ¹⁷⁷Lu (in s^-1), σ = thermal neutron capture cross section of ¹⁷⁶Lu at the neutron velocity of 2200 m.s^-1 (2090 b), σ’ = thermal neutron capture cross section of ¹⁷⁷Lu at the neutron velocity of 2200 m.s^-1 (1000 b), φ = thermal neutron flux of the reactor (in n.cm^-2.s^-1), t = time of irradiation and ‘k’ is so called k-factor, the value of ‘k’ is reported to be between 1.5-2.5 [7]. This expression of the yield of ¹⁷⁷Lu is based on the assumption that the neutron flux is highly thermalized and there is practically no contribution of epithermal neutrons towards ¹⁷⁷Lu formation. Based on Equation 1, the variation of theoretical yield of ¹⁷⁷Lu as a function of irradiation time at the irradiation position in Dhruva (where k = 1.75) is shown in Figure 2. It is evident for the figure that the yield is maximum (~21 Ci/mg of Lu) when irradiation is carried out for 13-14 days. Hence, Lu targets are irradiated for 2 weeks for production of ¹⁷⁷Lu in India. Also, it is very interesting to note that, in case of ¹⁷⁷Lu, there is a mismatch between the theoretically calculated specific activity and that actually obtained l from its yield per unit mass of the target irradiated. This is due to the significant burn up of target during the irradiation for which the actual mass of lutetium present in the system, post irradiation is significantly reduced compared to the initial mass of the target irradiated. Using the burn-up correction, the actual specific activity S (Bq/mol) of ¹⁷⁷Lu can be expressed as (Equation 2):

Figure 2.

Theoretical yield ¹⁷⁷Lu as a function of irradiation time at the irradiation position in Dhruva research reactor in India (ф = ~1.6×10¹⁴ n.cm^-2.s^-1, k = 1.75).

where, n₀ is the number of moles of the target isotope ¹⁷⁶Lu at the start of irradiation and n is the number of moles of other lutetium isotopes which do not lead to the formation of ¹⁷⁷Lu by (n, γ) process. Based on Equation 2, the specific activity of ¹⁷⁷Lu has been found to be 1.25 times higher compared to the yield of ¹⁷⁷Lu, when irradiations of Lu targets (~82% enriched ¹⁷⁶Lu) were carried out in Dhruva at a thermal neutron flux of 1.6×10¹⁴ n.cm^-2.s^-1 for 14 d. This enhancement of specific activity is an added advantage for ¹⁷⁷Lu towards its utilization in receptor specific therapeutic radiopharmaceuticals. Practically, the specific activity of ¹⁷⁷Lu has been found to be 900 ± 78 GBq/mg (24.3 ± 2.1 Ci/mg) (averaged over 300 batches).

The impurity burden of ^177mLu in ¹⁷⁷Lu produced in our case was found be ~0.015% of ¹⁷⁷Lu activity produce at end of irradiation (EOI) and ~0.02% of ¹⁷⁷Lu activity produced at 48 h post-EOI, when ¹⁷⁷Lu is generally used in the clinic. A typical gamma ray spectrum of [¹⁷⁷Lu]LuCl₃ used for formulation of [¹⁷⁷Lu]LuDOTATATE is shown in Figure 3, which confirms very high radionuclidic purity of the radiopharmaceutical precursor. This implies that for each patient administered with 7.4 GBq dose of ¹⁷⁷Lu-labeled-S-analogue peptide, the administered dose of ^177mLu is ~1.48 MBq (40 μCi). The presence of ^177mLu at this level is not a matter of serious concern from the point of view of radiation dose burden to the patients. However, the use of each GBq of ¹⁷⁷Lu will lead to the accumulation of ~200 kBq of radioactive waste of ^177mLu to be disposed by following delay and decay approach.

Figure 3.

Typical gamma-ray spectrum of [¹⁷⁷Lu]LuCl₃ used for the formulation of [¹⁷⁷Lu]Lu-DOTATATE.

Formulation of [¹⁷⁷Lu]Lu-DOTATATE for clinical use

The therapy doses of [¹⁷⁷Lu]Lu-DOTATATE radiopharmaceutical are formulated mainly in hospital radiopharmacy prior to clinical use following a protocol optimized after extensive radiochemical studies. Moderate specific activity [¹⁷⁷Lu]LuCl₃ produced (20-24 Ci/mg) indigenously by direct neutron activation route is used for the formulation. The general protocol followed for the synthesis of the radiopharmaceutical is presented schematically in Figure 4. During the last decade more than 400 batches of the [¹⁷⁷Lu]Lu-DOTATATE were synthesized (10-15 therapeutic doses in each batch) with >95% radiochemical purity, as confirmed by radio-HPLC (Figure 5). The specific activity of the formulation varied from 1.0 mCi/µg (53.1 GBq/µmol) to 1.2 mCi/µg (63.8 GBq/µmol) of DOTATATE. The specifications of the radiopharmaceutical formulation are summarized in Table 2.

Figure 4.

Schema of General protocol for the synthesis of [¹⁷⁷Lu]Lu-DOTATATE.

Figure 5.

Typical HPLC radio-chromatogram of [¹⁷⁷Lu]Lu-DOTATATE formulation. HPLC was carried out using a dual pump HPLC unit with a C-18 reversed phase HiQ-Sil (5 μM, 25 cm×0.46 cm) column. The elution was monitored both by detecting UV signals at 270 nm as well as by radioactivity signal using NaI(Tl) detector. Gradient elution with Water (A) and acetonitrile (B) mixtures with 0.1% trifluoroacetic acid were used as the mobile phase (0-5 min 90% A, 5-15 min 90% A to 10% A, 15-20 min 10% A, 20-25 min 10% A to 90% A, 25-30 min 90% A). Flow rate was maintained at 1 mL.min^-1.

Table 2.

Specifications of [¹⁷⁷Lu]Lu-DOTATATE therapy dose (~7.4 GBq, 200 mCi) formulation

Parameter	Specification
Appearance	Clear pale yellow solution without any suspended particulate
pH	~4.5
Volume	~1.0 mL
Radionuclidic purity	>99.95%
Radiochemical purity	>95.0%
Sterility	Sterile formulation
Apyrogenicity	Pyrogen content <25 IU/mL

PRRT: clinical services

The PRRT clinical services was initiated at the Radiation Medicine Centre (RMC) -Tata Memorial Hospital (TMH) premises in 2010, by the joint efforts of Radiation Medicine Centre (RMC), BARC (Bhabha Atomic Research Centre) and GI services of the Tata Memorial Hospital (TMH). More than 4,000 therapies have been successfully administered with [¹⁷⁷Lu]Lu-DOTATATE in around 1000 patients, and has involved patients of (i) Gastroenteropancreatic NENs (GEP-NENs), (ii) Metastatic NEN with unknown primary (CUP-NEN) (iii) Bronchopulmonary, Mediastinal and Thymic NENs, (iv) Medullary thyroid carcinoma (MTC), (v) non-[¹³¹I]I-MIBG concentrating metastatic Paraganglioma & Pheochromocytomas, and (vi) other miscellaneous tumors (non-iodine concentrating metastasis of differentiated thyroid carcinoma or TENIS, metastatic Merkel Cell carcinoma, Meningioma, recurrent/inoperable Phosphaturic Mesenchymal Tumor and sinonasal neuroendocrine carcinoma) (Table 3; Figure 6). The PRRT with [⁹⁰Y]Y-DOTATATE is relatively new addition that has started from September 2019 as a part of duo-PRRT protocol, which is at its optimization phase at present (detailed later). The important milestones in our clinical PRRT programme has been depicted in Figure 7. Under this sections, we have elaborated upon the learning points and have shared our perspectives on these themes or issues.

Table 3.

Spectrum of Malignancies treated by PRRT with ¹⁷⁷Lu-DOTATATE and their relative distribution

Patients characteristics	Number of Patients
177Lu-DOTATATE PRRT	1000
90Y-DOTATATE PRRT	11
Site of primary disease for 1000 pts
Gastroenteropancreatic NENs (GEP-NENs)	781
Metastatic NEN with unknown primary (CUP-NEN)	87
Lung NENs	30
Mediastinal/thymus NENs	27
Medullary Thyroid carcinoma (MTC)	50
Pheochromocytomas and Paraganglioma (PCC/PGL)	15
Miscellaneous	10

Figure 6.

Pie Chart of Spectrum of Malignancies treated by PRRT with [¹⁷⁷Lu]Lu-DOTATATE and their relative distribution.

Figure 7.

The important milestones in the Clinical PRRT Services and Practice Evolution in our set-up.

The clinical efficacy of ¹⁷⁷Lu obtained through “Direct” route as a therapeutic radiopharmaceutical: results in major malignancies

As mentioned before, two alternative strategies are available for production of ¹⁷⁷Lu with adequate specific activity required for its utility in PRRT. The first is the so called “direct” route which results in the production of carrier-added ¹⁷⁷Lu. The second route or the “indirect” route is based on neutron irradiation of highly enriched (in ¹⁷⁶Yb) ytterbium targets leading to the formation of no-carrier-added (NCA) ¹⁷⁷Lu from the β- decay of the short-lived activation product ¹⁷⁷Yb (T_1/2 = 1.9 h).

In India, more than 90% of the PRRT procedures (nearly all government centres and majority of the private centres) are undertaken using ¹⁷⁷Lu obtained via the first pathway indigenously in the research reactor. There has been some debate on the efficacy of ¹⁷⁷Lu produced through the direct route. At our Centre, we have obtained gratifying response with the indigenous product in large number of patients with clinical results similar to that is reported with the product from the ‘indirect’ route (two clinical examples depicted in Figures 8, 9, 10 and 11) in the literature [14-18]. There was excellent concordance of the ranges in tumor absorbed doses (estimated in metastatic NET patients during different PRRT cycles) with indigenous [¹⁷⁷Lu]Lu-DOTATATE and that by the ¹⁷⁷Lu produced through ‘indirect’ route (a comparative table of the tumor absorbed doses with the reported studies and our own experience has been illustrated in Figure 11; Table 4). Herein we present the comparative data with contemporary literature in the 3 most common malignancies where the [¹⁷⁷Lu]Lu-DOTATATE PRRT has been employed in our services viz. (i) Metastatic Advanced and Progressive Gastroenteropancreatic Neuroendocrine Tumors, (ii) Metastatic Advanced Pulmonary and Mediastinal Neuroendocrine Tumors, and (iii) Metastatic Medullary Carcinoma of thyroid.

Figure 8.

Post-PRRT complete response (CR) in a known patient of grade I (Ki-67 index = 1%) pancreatic NET with multiple hepatic metastases. The baseline [⁶⁸Ga]Ga-DOTATATE scan (A) showed intensely SSTR avid multiple liver lesions and pancreatic tail lesion. Patient received 3 cycles of [¹⁷⁷Lu]Lu-DOTATATE PRRT (cumulative dose of 550 mCi/20.35 GBq). The [¹⁷⁷Lu]Lu-DOTATATE post-therapy scan (B) showed good concentration of tracer in liver lesions and pancreatic lesion. Following 3 cycles of PRRT, follow-up [⁶⁸Ga]Ga-DOTATATE scan (C) showed complete resolution of liver lesions and pancreatic lesion suggestive of CR.

Figure 9.

Post-PRRT partial response (PR) in a case of grade II pancreatic NET (Ki-67 index = 5%) with liver metastases. Pre-PRRT [⁶⁸Ga]Ga-DOTATATE PET-CT (A) showed intensely tracer avid pancreatic lesion measuring 8×7 cm and few metastatic liver lesions. The patient received 5 cycles of PRRT with [¹⁷⁷Lu]Lu-DOTATATE (cumulative dose: 800 mCi/29.6 GBq). Post-PRRT, [⁶⁸Ga]Ga-DOTATATE scan (B) showed reduction in size of pancreatic lesion (4×3.7 cm) and liver lesions suggestive of partial response to PRRT.

Figure 10.

(A) 53 year old female, diagnosed as a case of atypical carcinoid of lung (MiB1 index of 6-10%), MiB-1 index of 6-10%. Multiple SSTR positive lesions in liver, both lungs, multiple rib & right sided pelvis, received 3 cycles of chemotherapy with cisplatin and etoposide with progression of disease. Excellent partial response was observed in the [⁶⁸Ga]Ga-DOTATATE PET-CT (A). The patient had a FDG negative disease at baseline (B). She reported a dramatic decrease in symptoms including decrease in abdominal pain and frequency of diarrhea. Also reported weight gain and overall improvement in general condition and health related quality of life.

Figure 11.

Illustration of comparison of tumor absorbed doses in different therapy cycle and a comparative table of the tumor absorbed doses with the reported studies and our own experience with indigenous [¹⁷⁷Lu]Lu-DOTATATE, demonstrating excellent concordance (Table 4).

Table 4.

Comparisons of tumor absorbed doses with literature data

Literature	Absorbed dose per unit activity (Gy/GBq)
Ulrike Garske et al EJNMMI 2018 [15]	2.8-5
Ezgi Ilan et al JNM 2015 [16]	1.35-45.95
Christiane W. et al Cancer Biother. & Radiopharm. 2007 [17]	3.0-15.1
Michael Garkavij et al Cancer, 2010 [18]	0.1-20
Our experience	1.15-20.5

Metastatic advanced and progressive gastroenteropancreatic neuroendocrine tumors

An interim analysis during 2014-2015 at our centre looked into 50 patients with histopathologically confirmed metastatic/inoperable GEP-NETs (M:F 33:17, age: 26-71 years) who had undergone at least three cycles of PRRT with [¹⁷⁷Lu]Lu-DOTATATE [19]. Symptomatic response was documented in more than 90% of patients in terms of improvement in health-related quality of life, the disease control rate (stable disease and partial response by scan) was observed in ~80% of cases. One point of note in this study was that (a) high pre-therapy FDG SUVmax values were associated with increased chances of treatment refractoriness (disease control rate 52.94% versus 96.97% in patients with absence of FDG uptake) and (b) symptomatic improvement was observed in most cases irrespective of tumor Ki-67 index (Table 5A and 5B). In a recent analysis of a total of 468 patients with proven GEP-NEN who underwent from 2 to 7 cycles (at the interval of 10-12 weeks) of intravenous [¹⁷⁷Lu]Lu-DOTATATE PRRT at our centre, the overall clinical results are further depicted partial response by PERCIST and RECIST 1.1 of 25% and 27% and stable disease of 57% and 60% respectively. The PFS at 7 yrs was 71.1% and OS was 79.4%. A total of 39 (8%) patients died out of 468 patients at 7 years (unpublished work; personal communication). The PRRT procedures were well-tolerated with minimal nephrotoxicity (grade 1: 3.5%, grade 2, grade 3 and grade 4: <1% each) and hematotoxicity primarily of grade 1 (1.7%), grade 2 and grade 3, 0.2% each and no grade 4 hematotoxicity (Figures 8 and 9).

Table 5A.

PRRT response in total 50 GEP-NETs on Symptomatic and PET-CT imaging [19]

Response	Symptomatic	18F-FDG/68Ga-DOTATATE PET/CT imaging responders
Yes (PR+SD)	96%	82%
No (PD)	4%	18%

Table 5B.

PRRT response in GEP-NETs with respect to SSTR-based and [¹⁸F]F-FDG PET/CT imaging findings [19]

[18F]F-FDG and [68Ga]Ga-DOTATATE PET/CT finding →	Discordance (SSTR>FDG avid lesions, n = 33)	Concordance (SSTR≤FDG avid lesions, n = 17)
PRRT response ↓
Yes (n = 41, 82%)	32/33 (96.97%)	9/17 (52.94%)
No (n = 9, 18%)	1/33 (3.03%)	8/17 (47.06%)

In a short series of metastatic NETs (both GEP-NEN and pulmonary NENs) with extensive bone marrow involvement at diagnosis, the majority (four out of five patients, i.e. 80%) of the patients had excellent symptomatic response with at least stabilization of the disease at a follow-up period of 10-27 months [20]. The single patient who had a progressive disease also had a good symptomatic response in the initial 6 months from the first dose of PRRT. Despite the extensive bone marrow involvement, no hematological toxicity was observed (only one patient showed Grade I anaemia), suggesting that PRRT was well-tolerated by this particular subgroup. We have obtained gratifying response and improvement of health related quality of life in patients with extensive metastatic disease (Figure 12).

Figure 12.

A 35 year old female, diagnosed patient of duodenal NET (MiB1 labeling index 20%) with liver metastasis (involving left lobe) and history of radiofrequency ablation of the liver metastasis and external radiotherapy six months previously, was referred for exploring the feasibility of PRRT. Despite such extensive disease, the patient started working at 4 years. (A) [^99mTc]Tc-HYNIC-TOC SSTR Imaging and (C) [¹⁷⁷Lu]Lu-DOTATATE post therapy scan showing tracer uptake in residual primary tumor in duodenum (dotted arrow) and liver lesion (thin solid arrow) with fairly large area of photopenia suggesting necrosis. Diffuse uptake over axial and appendicular skeleton (with interspersed areas of focal uptake, particularly in left parietal bone) and intense bilateral uptake in breast nodules (thick solid arrows) indicate metastases from NET. (B) [¹⁸F]F-FDG PET/CT demonstrating relatively intense uptake in duodenal primary tumor (dotted arrow), intense uptake bilaterally in metastatic breast nodules (solid arrows), low-grade [¹⁸F]F-FDG uptake in liver lesion, and no uptake in bone marrow. (Reproduced with permission [21]).

A large body of literature and guidelines exist today demonstrating efficacy of [¹⁷⁷Lu]Lu-DOTATATE in metastatic GEP-NENs [22-29]. Wang et al in a systematic review and meta-analysis of 22 studies (1758 patients) estimated an overall CR+PR of 35.0% and CR+PR+SD of 83% (on RECIST 1.1) [26]. Another systematic meta-analysis of 15 studies by Zhang et al reported a CR+PR of 27.58% on RECIST 1.1 and CR+PR+SD of 79.14% on RECIST 1.1 [27]. The PFS and OS were not estimated in both of these meta-analyses. Ezzidin et al [28] in their study of 74 patients with a median follow-up of 47 months, had documented a PFS and OS of 26 months and 55 months respectively, while that by Brabander et al in 443 patients, were 29 and 63 months respectively. Thus, the clinical results with the indigenously produced ¹⁷⁷Lu has been in agreement with what has been reported in the literature (Table 6) [29].

Table 6.

Meta-analysis and literature reports of response and survival of PRRT in metastatic GEP-NETs

Name/author of the study	No of studies/patients analyzed	Radionuclide used for PRRT	Follow up period after PRRT	Response on anatomical or molecular	PFS	OS
Wang LF et al [26] Systematic meta-analysis	22 studies (1758 patients)	177Lu and 90Y-DOTATATE	Review article	35.0% = CR+PR on RECIST 1.1	Not evaluated	Not evaluated
				83% = CR+PR+SD on RECIST 1.1
Zhang J et al [27] Systematic meta-analysis	15 studies	177Lu and 90Y-DOTATATE	Review article	27.58% = CR+PR on RECIST 1.1	Not evaluated	Not evaluated
				79.14% = CR+PR+SD on RECIST 1.1
Ezzidin et al [28]	74	177Lu-DOTATATE	47 months (median follow up)	36.5% PR, 17.6% MR, 35.1% SD, and 10.8% PD	26 mo	55 mo
Brabander et al [29]	443	177Lu-DOTATATE	Not mentioned	SD-36%	29 mo	63 mo

Metastatic advanced pulmonary and mediastinal neuroendocrine tumors

Metastatic pulmonary and mediastinal neuroendocrine tumors form the next tumor group that is considered for the [¹⁷⁷Lu]Lu-DOTATATE peptide receptor radionuclide therapy [PRRT] in our setting. In an initial analysis in metastatic/advanced pulmonary (NETs) in 19 patients, symptomatic response following PRRT was observed in 79%. A total of 63% were finally characterized as responders and 37% were overall non-responders to PRRT based upon predefined 3-scale response evaluation criteria. All 7 non-responders had moderate to intense FDG-avid primary lung lesion and 5 had FDG-avid metastatic liver disease (SUVmax >5) (Table 7). The median OS was 40 months [30]. The results obtained were well-commensurate with two studies reported in literature on ¹⁷⁷Lu-octreotate in metastatic pulmonary neuroendocrine tumors, where the progressive disease reported were 32% and 30%, one in pure metastatic pulmonary NET scenario [31] and the other in a combined analysis of patients with gastroenteropancreatic and bronchial neuroendocrine tumors [29]. In the study by Sabet et al, the DCR was 67% (PR = 27%, SD = 41% and CR 0%) matches well with the responder group of 63% in our preliminary analysis [30,31]. Response evaluation to PRRT in 30 patients of metastatic pulmonary NETs over last 10 years shows a PD of 31% on RECIST 1.1 and 47% on PERCIST & biochemical parameters (unpublished work; personal communication). Thus, the DCR (SD+PR+CR) by RECIST 1.1 was 69% and by PERCIST & biochemical parameters was 53% (Table 8; Figures 10 and 13).

Table 7.

Response evaluation to PRRT in 19 pulmonary NET patients [30]

Response	Symptomatic evaluation	Biochemical evaluation	PERCIST*	RECIST 1.1**
Complete response	42%	7%	5%	5%
Partial Response	26%	15%	10%	5%
Minor response			20%	20%
Stable disease	12%	31%	18%	37%
Progressive disease	20%	47%	47%	33%

^*PERCIST=PET response criteria in solid tumors.

^**RECIST 1.1=Response Evaluation Criteria In Solid Tumors.

Table 8.

Response evaluation to PRRT in 30 patients of metastatic pulmonary NETs over last 10 years

Response	Symptomatic	Biochemical	RECIST 1.1	PERCIST
CR	42%	7%	4%	5%
PR	26%	15%	8%	12%
SD	12%	31%	57%	36%
PD	20%	47%	31%	47%

Figure 13.

A. Patient of Pulmonary NET with hepatic metastases, presenting with profound anorexia and weight loss over previous 2 months. The baseline [⁶⁸Ga]Ga-DOTATATE (a) and FDG PET-CT in May 2016 show tracer avid left lung mass and multiple liver lesions with minimal to absent FDG uptake in the lesions (b). The patient received 5 cycles of PRRT from June 2016 to Dec 2017 (750 mCi/27.75 GBq). B. In 2018, the patient was asymptomatic with decrease in serum CgA level and [⁶⁸Ga]Ga-DOTATATE PET-CT showed no changes in size, number of lesions or tracer uptake in the primary left lung lesion and metastatic liver lesions, with increase in necrotic changes in liver and lung lesions-suggesting disease stabilization in this case.

With regard to mediastinal NETs, twenty-seven patients with histopathologically and radiologically proven metastatic or advanced mediastinal NET, who had undergone [¹⁷⁷Lu]Lu-DOTATATE PRRT, metabolic PET-CT response evaluation demonstrated partial metabolic response in 33.3%, metabolically stable disease was seen in 18.5%, while 44.4% of the patients showed metabolic disease progression (Table 9; Figure 14). The median PFS and OS were 36 months and 66 months respectively. Significant level of association of PFS was observed with surgical intervention, higher cumulative PRRT dose, metabolic response, smaller sized primary lesion, and low lesional FDG uptake; with respect to the OS, higher cumulative PRRT dose, low FDG uptake and longer PFS showed significant association [32]. There is relatively sparse clinical study data in the literature till date on [¹⁷⁷Lu]Lu-DOTATATE PRRT in metastatic mediastinal/thymic NETs, which are primarily reported as case reports.

Table 9.

Response evaluation results of Mediastinal NETs to [¹⁷⁷Lu]Lu-DOTATATE PRRT [32]

Response	Symptomatic	Metabolic	Anatomical
CR	37%	3.7%	0%
PR	25.9%	33.3%	22.2%
SD	14.8%	18.5%	25.9%
PD	22.2%	44.4%	51.9%

Figure 14.

¹⁷⁷Lu-DOTATATE PRRT in disease stabilization of Thymic NET. The patient was diagnosed case of MEN 1, with thymic NET (Mib-1 index = 15%), pituitary adenoma and parathyroid adenoma. He was operated for pituitary adenoma, parathyroid adenoma and inoperable for thymic NET. He was evaluated for PRRT in view of inoperable and metastatic disease. [⁶⁸Ga]Ga-DOTATATE and FDG-PET/CT showed SSTR and FDG avid enlarged mediastinal lesions (A), patient received 2 cycles of PRRT (B), the follow-up [⁶⁸Ga]Ga-DOTATATE and FDG PET/CT in Dec 2017 (C) demonstrates stable disease.

We have to mention here that for both metastatic pulmonary and mediastinal NENs, in addition to the aggressive biology of this tumor group, PRRT has been usually considered at an advanced state following chemotherapy failure which we believe could be partly responsible for its relatively inferior outcome compared to the GEP-NENs.

Metastatic medullary carcinoma of thyroid (MTC)

[⁶⁸Ga]Ga-DOTATATE positive metastatic MTC forms the 3^rd most common indication for PRRT in our set-up. In a total of 43 somatostatin receptor-positive metastatic MTC patients treated with [¹⁷⁷Lu]Lu-DOTATATE PRRT [33], 61% were responders and 39% were nonresponders based upon PERCIST criteria, while with RECIST 1.1 criteria 62% were responders and 38% were non-responders (Figure 15). Significant association between responses to PRRT was observed with following prognostic variables: (a) size of lesions (<2 cm) and (b) FDG uptake in lesions (SUVmax of <5). PRRT was well tolerated in all patients without any major grade 3 or 4 toxicity. The median OS was 26 months and the median PFS 24 months. There are much smaller series that have been published in this domain and more results may be awaited for comparison with the indigenous [¹⁷⁷Lu]Lu-DOTATATE (Table 10) [34,35].

Figure 15.

Metastatic MTC treated with [ ¹⁷⁷ Lu]Lu-DOTATATE PRRT. Diagnosed case of MTC with past history of total thyroidectomy and bilateral selective neck dissection presents with follow-up serum calcitonin of 6271 pg/ml and CT abdomen showing metastatic liver lesions. Baseline [⁶⁸Ga]Ga-DOTATATE PET-CT (A) showed intensely SSTR avid (Krenning score = 4) bilobar liver lesions (SUVmax 25) and bony metastasis in right acetabulum (SUVmax 23). Subsequently, the patient received 2 cycles of PRRT (300 mCi). ¹⁷⁷Lu-DOTATATE post therapy scan (B) showed good concentration of tracer in liver and skeletal lesions. Post-PRRT [⁶⁸Ga]Ga-DOTATATE PET-CT (C) showed reduction in SSTR uptake in liver lesions (SUVmax 16) and right acetabulum bony lesion (SUVmax 13) with significant reduction serum calcitonin level from 6271 to 2167 pg/ml, suggestive of favorable response to PRRT.

Table 10.

PRRT response evaluation in MTC patients in four categories [33]

Response	Symptomatic	Biochemical	RECIST 1.1	PERCIST
CR	19%	11%	0%	0%
PR	28%	30%	4%	10%
SD	4%	10%	58%	51%
PD	49%	49%	38%	39%

Dual tracer PET-CT in individualizing treatment regimen in NENs: a valuable adjunct alongwith tumor Ki-67/MIB-1 index in assessing tumor biology

While the tumor Ki-67/MIB-1 labeling index forms an integral part of assessment of metastatic NENs, dual tracer PET-CT molecular imaging findings (somatostatin receptor based [⁶⁸Ga]Ga-DOTATATE and tumor glucose metabolism with FDG) has developed into a valuable adjunct for personalizing the treatment regimen in this group of patients (Figure 15). Several studies in the early years including ours had observed that tumors with low or negligible FDG uptake respond well to somatostatin targeted therapy, while those with high FDG uptake demonstrates refractoriness to PRRT alone [19,37]. The relative uptake of [⁶⁸Ga]Ga-DOTATATE and FDG enables assessment of the dynamic tumor biology of NENs on a continuous scale and is utilized for clinical decision making and personalizing the treatment strategies [38]; depending upon the dual tracer PET-CT findings, the patients are subdivided into 3 classical subgroups: (i) Tumors with low Ki-67 index, are usually positive on SSTR imaging with low/absent FDG uptake: they are candidates for SSA and PRRT, (ii) Tumors with high Ki-67 index, are usually negative on SSTR based imaging and shows high uptake on FDG-PET/CT: are candidates for chemotherapy (CAPTEM if the Ki-67<55% or platinum based chemotherapy if the Ki-67>55%) (Figure 16). (iii) Tumors demonstrating both high [⁶⁸Ga]Ga-DOTATATE and ¹⁸F-FDG uptake on dual tracer PET/CT form the intermediate grey zone and intermediate Ki-67 index, wherein a combined approach of PRRT plus chemotherapy is employed (Figures 17, 18 and 19) [36,38,39].

Figure 16.

Dual tracer PET-CT as valuable adjunct in personalizing treatment regimen in metastatic NENs (Reproduced with permission from Basu et al [36]).

Figure 17.

[⁶⁸Ga]Ga-DOTATATE and [¹⁸F]F-FDG PET-CT in a patient of CUP-NET with Ki-67 index = 2% showing matched histopathology and dual tracer molecular PET-CT imaging finding with minimal FDG and avid [⁶⁸Ga]Ga-DOTATATE uptake.

Figure 18.

A known case of NEC with unknown primary site. Presenting with complaints of abdominal pain and cough, the biopsy from abdominal lesions showed metastatic carcinoma with neuroendocrine differentiation and positivity for CK7, synaptophysin and negativity for chromogranin and p40. Ki-67 index was 80-90%. The patient did not receive any treatment before the molecular imaging. [¹⁸F]F-FDG PET-CT scan (A) showed intensely FDG avid multiple liver lesions, lung nodules, skeletal lesions and mesenteric deposits with no uptake in above-mentioned lesions on [⁶⁸Ga]Ga-DOTATATE PET-CT (B). The patient is NOT a suitable case for PRRT.

Figure 19.

Dual tracer PET finding matched with that predicted by HPR. Pancreatic NET with liver lesions, Ki-67 index = 15%, showing high uptake on both [⁶⁸Ga]Ga-DOTATATE (A) and FDG (B) PET-CT. The patient is a typical case for consideration for combined chemo-PRRT.

The specific areas where this manoeuvre becomes particularly useful for clinical decision making are:

(i) The tumor biology in tumors with intermediate level of Ki-67 labeling index (e.g. 10-20%) and also those with Ki-67 LI between 20-30%

Wide variation in tumor behaviour at both of the above regions of Ki-67 index, where the relative uptake of [⁶⁸Ga]Ga-DOTATATE and FDG becomes particularly useful for deciding on the therapy on an individualized basis. Having observed this early-on, we have adopted the practice routinely in our Institute.

The observation made over years have been recently reflected by the WHO 2017 NEN grading. Amongst the grade 3 NENs, there is now increasing recognition of a ‘well differentiated NEN composed of cells showing minimal to moderate atypia, lacking necrosis and expressing general markers of neuroendocrine differentiation (diffuse and intense synaptophysin and chromogranin A)’ which contrasts to the ‘poorly differentiated NEN is composed of highly atypical small or large cells expressing faint neuroendocrine differentiation markers’. Hence, two different subsets within the grade 3 NENs are now recognized by the recent WHO 2017 NEN classification, distinguishing well differentiated grade 3 neuroendocrine tumours (NET G3) from poorly differentiated grade 3 neuroendocrine carcinomas (NEC G3), which is an important step from management view-points.

Dual tracer PET-CT findings of the relative tracer accumulation in tumor lesions based upon somatostatin receptor expression (by [⁶⁸Ga]Ga-DOTATATE/NOC PET-CT) and tumor glycolysis (by FDG-PET/CT), can potentially aid in deciphering the aforementioned tumor characteristics, and differentiating NET G3 from NEC G3. A high [⁶⁸Ga]Ga-DOTATATE/NOC with low/absent FDG uptake would be more consistent with the diagnosis of G3-NEC.

(ii) Discordance between Ki-67/MIB-1 and the dual tracer PET-CT findings

Though the standard findings on dual tracer PET-CT is characteristic and remains in agreement with the reported Ki-67/MIB-1 labeling index, the outliers or exceptions are not uncommon in day-to-day practice. Herein two considerations need to be made: (a) Ki-67/MIB-1 labeling index from a single lesion and single-site biopsy could be fraught with its inability to assess the tumor biology as a whole on a continuous scale and (b) even in the same individual, interlesional heterogeneity can exist, thus one biopsy specimen may not be representative of all lesions, which could be better portrayed by the molecular PET-CT imaging (Images of case examples: Figures 20, 21). In our set-up the findings in the PET-CT imaging forms an important parameter and given equivalent important to the Ki-67 index for clinical decision-making of therapies in metastatic NENs, if not more.

Figure 20.

Discordance between Ki-67 and dual tracer PET-CT findings in a patient of metastatic NET of unknown primary site, liver biopsy reported as metastatic NET with Ki-67 index = 22%. [⁶⁸Ga]Ga-DOTATATE PET-CT (A) showed intensely SSTR avid multiple liver and wide spreads skeletal lesions. [¹⁸F]F-FDG PET-CT (B) showed mild FDG uptake in liver lesions and faint FDG uptake in skeletal lesions.

Figure 21.

Discordance between Ki-67 and dual tracer PET-CT findings in a patient of pancreatic NET with pancreatic mass involving the body and tail with splenic and colonic infiltration. HPR:NET with Ki-67:1-2%. However, shows low grade SSTR based ⁶⁸Ga-DOTATATE uptake (left panel) but high on FDG (right panel) PET-CT.

PRRT beyond classical indication of GEP-NENs: expanding the boundaries of SSTR based Theranostics

The classical standard indications for PRRT has been (i) metastatic and unresectable advanced grade I or II gastroenteropancreatic neuroendocrine neoplasms (GEP-NENs) that are (ii) progressive on Synthetic Somatostatin Analogues (SSAs) and demonstrate high grade uptake (Krenning score 3 or 4 on semi-quantitative scale) on SSTR based [⁶⁸Ga]Ga-DOTATOC/TATE PET-CT or [^99mTc]Tc-HYNIC-TOC SPECT-CT. (iii) Symptomatic functioning NENs, who are resistant to the long-acting SSAs (octreotide/lanreotide) form another group where PRRT is usually considered. Beyond the aforementioned indications, the clinical applications of PRRT has been widened substantially over the recent years, and can be subdivided into two groups: (A) extended indications within GEP-NENs and (B) applications in tumors beyond GEP-NENs.

Extended indications within GEP-NENs

The following two groups of GEP-NEN patients are now frequently considered for PRRT with ¹⁷⁷Lu-DOTATATE and increasingly becoming clinical routine: (iv) In NENs with MIB-1 index between 20 and 30%, a sizeable fraction of patients demonstrate high uptake on [⁶⁸Ga]Ga-DOTATATE PET-CT, where PRRT can be successfully employed [40]. Amongst this subgroup, a fraction of patients do show high uptake of FDG additionally, wherein we advocate combined chemo-PRRT (the regimen employed in our set-up is detailed later). The ESMO clinical practice guidelines for GEP-NENs advocates PRRT upto Ki-67 of 30%.

(v) While usually PRRT is considered in patients with disease progression on cold somatostatin analogues, there has been increasing adoption of this treatment modality upfront at diagnosis in patients with widely metastatic disease on whole body diagnostic SSTR based study, where the disease is already ‘progressed’ involving multiple sites.

Tumors beyond Gastroenteropancreatic NENs (GEP-NENs)

Over the years, we have now acquired fair amount of experience in various ‘beyond GEP-NENs’ indications, wherein ¹⁷⁷Lu-DOTATATE PRRT provide good disease stabilization in substantial fraction of patients. These include: (a) metastatic/inoperable Bronchopulmonary and Mediastinal/Thymic NENs, [30,32]; (b) metastatic/inoperable Medullary thyroid carcinoma** [33]; (c) Non-¹³¹I-MIBG concentrating metastatic Paraganglioma & Pheochromocytoma (Figure 22); (d) Non-iodine concentrating metastasis of differentiated thyroid carcinoma (TENIS: only 15-20% of this patient subgroup demonstrates enough uptake to justify PRRT)**, [41]; (e) Other tumors with neuroendocrine tumor differentiation/characterization: metastatic Merkel Cell carcinoma, Meningioma and recurrent/inoperable Phosphaturic Mesenchymal Tumor, Recurrent Metastatic Sinonasal Neuroendocrine Carcinoma, Recurrent metastatic large cell neuroendocrine carcinoma (LCNEC) of larynx (Figure 23) [42-46].

Figure 22.

Diagnosed patient of invasive paraganglioma of carotid body, Mib-1 index 10%, the patient underwent excision of right carotid body tumor in 2013 and received post-operative EBRT in 2013. Presented with bony pain and headache in 2016. Patient evaluated for PRRT, the [⁶⁸Ga]Ga-DOTATATE (B) and FDG PET/CT (A) showed multiple skeletal lesions and lung nodules, following which the patient received 2 cycles of PRRT (C). In 2017 both [⁶⁸Ga]Ga-DOTATATE (D) and FDG PET/CT show SSTR and FDG avid lesions with no change as compared to baseline-suggesting stable disease (SD) in metastatic paraganglioma with PRRT.

Figure 23.

Observation of [⁶⁸Ga]Ga-DOTATATE (A) avid metachronous Meningioma in a known patient of pancreatic NET (Ki-67 index = 4%) with metastatic liver lesions. Initially the patient underwent right hepatectomy and pancreaticoduodenectomy in 2015. In subsequent years, he complained of abdominal pain, weakness and headache. ⁶⁸Ga-DOTATATE PET-CT showed SSTR avid brain lesion (B, blue arrows), liver lesion (C), skeletal lesion (D), abdominal lymph nodal and lung lesions (E). Meningioma was confirmed on MRI of brain region.

‘**’ represent clinical indications where PRRT has been considered even though there was a lesser degree of uptake (Krenning score 2) on SSTR based scanning, in view of the fact that alternative experimental therapies were either potentially toxic, only modest efficacious or expensive. The aforementioned list of PRRT indications is partly adapted with newer additions from Basu et al [46]).

The role of [^99mTc]Tc-HYNIC-TOC SPECT/SPECT-CT as decision making scan in the absence of [⁶⁸Ga]Ga-DOTATOC/NOC/TATE PET-CT

The diagnostic modality of choice for SSTR based imaging is [⁶⁸Ga]Ga-DOTATOC/NOC/TATE PET-CT, which has been proven superior to the other conventional diagnostic modalities including the other whole body functional imaging (such as [¹¹¹In]In-octreotide or [^99mTc]Tc-HYNIC-TOC SPECT-CT), in view of its superior resolution and ability for quantification.

However, in the absence of PET-CT or ⁶⁸Ge/⁶⁸Ga generator, the mandatory requirements for [⁶⁸Ga]Ga-DOTATOC/NOC/TATE PET-CT, a factor to be considered in several peripheral centres in developing nations, [^99mTc]Tc-HYNIC-TOC planar and SPECT-CT gamma camera based imaging can do reasonably well for decision making of PRRT in patients with large volume disease. In India, [^99mTc]Tc-HYNIC-TOC is available for kit formulation method which played important role in the initial years of PRRT development at our set-up (Figures 24, 25) [47].

Figure 24.

Pancreatic NET with no metastatic disease showing concordance between [^99mTc]Tc-HYNIC-TOC (A) and [⁶⁸Ga]Ga-DOTATATE PET/CT (B).

Figure 25.

Jejunal NET with metastatic disease in abdominal lymph nodes, liver and skeleton. [⁶⁸Ga]Ga-DOTATATE PET-CT (B) has advantage in detecting small volume/tiny lesions as compared to [^99mTc]Tc-HYNIC-TOC planar study (A). SPECT-CT enhances the lesion detectability.

Patient preparation in specific clinical scenarios

Large volume functioning hepatic metastases: minimizing the incidence of ‘carcinoid crisis’

Overall, PRRT is a very well-tolerated procedure with minimal adverse effects, however, post-PRRT acute carcinoid syndrome (“carcinoid crisis”) is a rare but life-threatening risk that requires emergency medical management. This is typically observed in patients with large volume functioning hepatic metastases. In our experience, adequate patient preparation is key to minimize the incidences in these cases, which would involve short acting octreotide injections (subcutaneous) till 1 day before of PRRT and initiate the same next day following PRRT continuing for 2 weeks. Priming this subgroup of patients with cyproheptadine on a regular basis 1 to 2 days before scheduled PRRT is also a helpful practice (it exerts its action as a potent antagonist of the 5-HT₂ receptors).

Protein deficiency, poor nutrition status and generalized anasarca: enhancing the chances of undertaking PRRT

This is a challenge to the attending physicians, more relevant to the developing world, the protein deficiency being primarily due to poor nutritional status, gastrointestinal loss of protein and tryptophan and hepatic dysfunction due to large hepatic disease burden. Oral protein supplementation along with octreotide therapy enhances the general condition and enables to administer PRRT in the isolation ward.

For malnourished patients with anasarca, undergoing treatment, generally a high calorie & high protein diet is recommended. In patients with conditions like ascites and pedal edema, the energy requirement varies between 25-40 kcals/kg/day but if the patient is underweight: it would be 35-40 kcals/kg/day. The protein requirements is higher for these patients due to nutritional hypoalbunemia (1.2-2 g/kg/bw). Supplement amount depends on patient’s present condition and food intake. The greater the decrease in food intake the higher the quantity of prescribed supplement. If the patient is on oral diet, lesser quantity is prescribed like 100 gms a day or lesser but the quantity is more if he is on tube feeds.

Use of aprepitant as an additional antiemetic during PRRT

Nausea and vomiting is a common occurrence during PRRT primarily related to the metabolic acidosis due to the amino acid co-infusion used for nephroprotection, rather than the radiopharmaceutical. This could be Dexamethasone-ondansetron pre-treatment on a regular basis, however, there remains a small fraction of patients who show refractory vomiting during or soon after the therapy which could be well managed with oral Aprepitant, an NK₁ antagonist in the central and peripheral nervous system, the typical adult dose is 125 mg, 80 mg and 80 mg on days 1, 2, and 3 respectively. In a patient who develops refractory vomiting during the first cycle of PRRT, he is considered for Aprepitant on a routine basis from 2^nd cycle onwards.

Carcinoid heart disease: can PRRT be potentially helpful in enhancing the chances of corrective vulvular surgery?

Carcinoid Heart Disease is seen in the setting of high volume functioning disease, typically in patients with bulky hepatic involvement which is an important risk factor (Peak 5-HIAA forms a significant predictor of the disease progression) that leads secretion of vasoactive substances (5-hydroxytryptamine, tachykinins, and prostaglandins) reaching the right side of the heart through hepatic veins and consequent deposition of fibrous tissue on the endocardial surfaces of the heart resulting in pathologenesis of carcinoid heart disease. Though the usual reported incidence is upto 70% within the subgroup of patients with carcinoid syndrome, its incidence is now much reduced owing to widespread use of synthetic somatostatin analogues (SSA) and better control of the functioning disease. We have some limited experience of managing such patients with resistant disease, where [¹⁷⁷Lu]Lu-DOTATATE helped in substantial reduction of 5-HIAA levels and resulted in symptomatic improvement and improved health related quality of life (from NYHA grade III at baseline to NYHA grade I after 4-6 cycles), finally enabling taking the patient for corrective valvular surgery [48].

One concern on advocating PRRT in patients of carcinoid heart disease has been the concern of volume overload in patients with features of cardiac insufficiency related to amino acid infusion and hence caution exerted in various guidelines [24]. Our Institutional protocol of a relatively prolonged mixed amino acid protocol of 7.5-8 hours adopted during PRRT is particularly helpful to address this concern, and can be recommended in this clinical setting of CaHD to undertake this treatment uneventfully. Furthermore, it needs to be mentioned that the renal absorbed dose has been reported to be further reduced by prolonging the infusion time of the amino acid solution over 10 hours by up to a factor of 39% [24].

PRRT in neoadjuvant setting

[¹⁷⁷Lu]Lu-DOTATATE PRRT shows modest success in the neoadjuvant setting, where the primary intent is reduction of the size of an initially unresectable primary GEP-NENs to bring to the point when it becomes operable. We follow a slightly different regimen in this context, where a higher mean dose with shorter time interval is followed (i.e. 200 mCi at 8 weekly interval) compared to the metastatic setting, where the intent is primarily disease stabilization and palliative (mean of 150 mCi per cycle administered at 12 weekly intervals for 5 such cycles). We have observed excellent tolerability of this regimen in all patients without any major hematologic or renal toxicity.

In our initial experience, the inoperable disease became operable in around 26% of patients, assessed in a population of 57 (Figure 26). This is similar to the published results, the success for operability is around one-third of treated patients (~30%) [49,50] (unpublished work; personal communications). Another observation in a sizeable fraction of patients that could be made with neoadjuvant PRRT is substantial regression of liver disease, thereby facilitating surgical removal of primary and residual liver lesions. This has been also reported by investigators who have employed [⁹⁰Y]Y-DOTATOC and [⁹⁰Y]Y-DOTATATE in the neoadjuvant setting [51,52].

Figure 26.

Efficacy of PRRT in neoadjuvant setting. A 35 years old male, diagnosed case of pancreatic NET with Ki-67 of 1%. Baseline [⁶⁸Ga]Ga-DOTATATE PET-CT scan (A) showed intensely SSTR avid (krenning score = 4) pancreatic head mass >180° contact with SMV (SUVmax 59, 5.6×4.0×7.2 cm) and SSTR avid peri-pancreatic lymph node (SUVmax 55, 3.4×2.6 cm). Subsequently, patient received 2 cycles of PRRT (400 mCi). ¹⁷⁷Lu-DOTATATE post therapy scan (B) showed good concentration of tracer activity in pancreatic head mass and peri-pancreatic lymph node. Follow-up [⁶⁸Ga]Ga-DOTATATE PET-CT scan (C) showed reduction in size and SSTR uptake of pancreatic head mass with <180° contact with SMV (SUVmax 34, 4.4×3.5×4.1 cm) and also reduction in SSTR uptake and size of peri-pancreatic lymph node (SUVmax 30, 2.3×1.7 cm), suggestive of favorable response to PRRT.

Sandwich Chemo-PRRT in patients with avid [⁶⁸Ga]Ga-DOTATATE and [¹⁸F]F-FDG uptake on dual tracer PET-CT: its comparison with PRCRT

From our initial years of experience with PRRT, we had observed that these group of tumors usually demonstrate aggressive biology, treatment refractoriness to PRRT alone and increased incidence of progressive disease and mortality in due course. Since 2013, we had adopted a combination approach (oral chemotherapy with capecitabine-temozolamide and [¹⁷⁷Lu]Lu-DOTATATE PRRT) for these group of patients. In chemo-PRRT protocol, 2 cycles of CAPTEM chemotherapy is sandwiched between 2 PRRT cycles of [¹⁷⁷Lu]Lu-DOTATATE.

The standard CAPTEM regimen comprises of oral capecitabine (CAP), 750 mg/m² twice daily for 14 days (D1-D14) and oral temozolomide (TEM) 200 mg/m² once daily for 5 days (D10-D14) followed by two weeks rest period and another CAPTEM cycle given for total 28 days is followed by next cycle of PRRT at around 3 months (Figure 27). In a population of 38 patients with metastatic NENs with high uptake on both ⁶⁸Ga-DOTATATE and ¹⁸F-FDG dual tracer PET/CT, who were treated with this Chemo-PRRT regimen, we found encouraging results with partial response in around 45%, stable disease in 39% and progressive disease in 16% on RECIST 1.1 with an overall disease control rate of 84% in this aggressive group of tumors (unpublished work; personal communications) (Figure 28).

Figure 27.

Protocol adopted for Sandwich Chemo-PRRT.

Figure 28.

Efficacy of ‘Sandwich Chemo-PRRT’. A diagnosed case of pancreatic NET (Ki-67 index = 8%) with hepatic and ovarian metastatic disease. The baseline [⁶⁸Ga]Ga-DOTATATE (A) and [¹⁸F]F-FDG PET-CT scans (B) in 2015 showed both SSTR and FDG avid multiple liver lesions, abdominal lymph nodes, bilateral adnexal lesions. The patient received sandwich Chemo-PRRT from 2015 to 2018. Post chemo-PRRT follow up [⁶⁸Ga]Ga-DOTATATE and [¹⁸F]F-FDG PET-CT scans in 2018 showed significant reduction of SSTR and FDG uptake in liver lesions, abdominal lymph nodes and bilateral adnexal lesions suggestive of PR after sandwich Chemo-PRRT treatment regimen.

Amongst the various combination regimen, PRCRT using ¹⁷⁷Lu-octreotate with concomitant 5-FU radiosensitizing infusional chemotherapy has been employed at other centres [53,54] in patients with FDG-avid NET. The dosage of 5-FU in PRCRT in this regimen has been (200 mg/m²/24 h), starting approximately 4 days before the day of PRRT administration, and continued for 3 weeks in total and early discontinuation in the event of hand-foot syndrome or other acute toxicity. There was achievement of good partial response, with long progression-free survival of 48 months in a cohort of 52 patients [54]. The other investigators have used (i) First 14 days of capecitabine 1500 mg/m² and last 5 days of temozolomide 200 mg/m² commencing on the morning of PRRT [55], (ii) oral capecitabine (1250 mg/m²) for 15 consecutive days from the 1^st day of PRRT [56], and (iii) oral capecitabine (500-1000 mg/m²) twice daily for 14 days (starting 1 day after the PRRT) and oral temozolomide (150-250 mg/m²) once per day for 5 days (days 10-14 after the PRRT) [57], with varying partial response (from 9% to 70%) being documented [55-57]. We believe that our aforementioned “sandwich chemo-PRRT” regimen could be more user friendly and easier to adopt in a busy clinical setting.

Duo-PRRT with [⁹⁰Y]Y-DOTATATE in combination with [¹⁷⁷Lu]Lu-DOTATATE in large volume lesions

The superior efficacy of [⁹⁰Y]Y-DOTATATE for large sized lesions and heterogeneous lesions results from higher beta-particle energy of 2.28 MeV (mean energy of 0.94 MeV) of ⁹⁰Y (maximum soft tissue range ~11 mm) compared to that of ¹⁷⁷Lu [E_β(max) = 0.497 MeV, maximum soft tissue range ~2.5 mm].

With the availability of indigenous [⁹⁰Y]Y-DOTATATE, 11 NET patients with large volume lesions have received [⁹⁰Y]Y-DOTATATE PRRT (Figure 29). Post PRRT stable disease has been observed in all 11 patients in the limited period of time, on imaging response evaluation with no major acute adverse effects. We are in the process of optimizing the duo-PRRT regimen at our set-up (with different regimen for the metastatic and neoadjuvant settings), including optimization of post-therapy Bremsstrahlung and PET-CT imaging following [⁹⁰Y]Y-DOTATATE treatment (Figures 29 and 30) [58,59].

Figure 29.

Treatment Protocol for “duo-PRRT” protocol for large volume disease (lesion size >5 cm) in metastatic and advanced Neuroendocrine Neoplasms (Reproduced with permission from Basu et al [42]).

Figure 30.

Post [⁹⁰Y]Y-DOTATATE whole body planar bremsstralung image (A) and post [⁹⁰Y]Y-DOTATATE regional PET-CT scan (B) in a known case of large sized pancreatic NET with liver metastatic disease (Ki-67 index = 1%), who had stable disease to [¹⁷⁷Lu]Lu-DOTATATE PRRT, showing good tracer uptake in primary pancreatic and metastatic liver lesions.

Radiation safety aspects in [¹⁷⁷Lu]Lu-DOTATATE PRRT

In our institute PRRT is administered as in-patient therapy procedure. World-over, there is some variation in this regard, while some countries consider PRRT as an outpatient procedure and some countries consider it as in-patient procedure. To minimise the radiation doses of staff, carers and general public each institute have their own radiation safety protocols. Our institute also have evolved elaborate radiation safety protocols, the salient points among them are mentioned in this review. Prior to administration of the [¹⁷⁷Lu]Lu-DOTATATE, patients and their relatives were provided with radiation safety and hygiene practices instructions. Patients were explained about their stay in special (isolation) ward, because this physical preparation of patients helps in minimizing the radiation exposure and contamination of physicians, nursing staff, other staff members and visitors. To avoid the spread of contamination, entry to the isolation ward is restricted. Standard operating procedures are mandated to be followed by the physicians, radiation safety officers, radio-pharmacists and nursing staff, during synthesis of [¹⁷⁷Lu]Lu-DOTATATE, preparation of patient doses, administration of [¹⁷⁷Lu]Lu-DOTATATE and discharge of patients from isolation wards. Regular radiation surveys and contamination checks are performed to ensure the limits set by the regulatory body are adhered. In our institute, all the liquid waste generated in isolation wards, including urine and faeces of NET patients are collected in dual delay tank. Once activity levels decrease to discharge limits set by our national regulatory body, discharges are let to municipal sewerage system.

To establish the discharge criteria and ensure that public exposure is less than the limit set by national regulatory body radiation field from the patient’s body should be monitored. In literature [60] it reported that the average exposure at one meter immediately after treatment is 19 ± 5 µSv/h (for mean administered activity 7.09 GBq) and at time of discharge (4-5 h) is 9.8 ± 3.1 µSv/h. In our experience we found that the average exposure at one meter immediately after treatment is 20.22 ± 3.21 µSv/h (for mean administered activity 7.4 GBq) and at time of discharge (18-20 h) is 7.05 ± 2.94 µSv/h (Table 11). The predictive effective dose to the family members of NET patients is found to be less than 0.15 mSv (Occupancy factor 0.5) [61]. Before discharging the patients from the ward, once again all instruction of radiation safety and hygiene practices were provided in written as well as verbal, so as to minimise the dose to family members and general public.

Table 11.

A comparison of Radiation Dosimetry and related parameters obtained with [¹⁷⁷Lu]Lu-DOTATATE through direct neutron activation route with that of reported literature

Parameters	Types	Our Study	Literature Value	References
Dose Ratea (µSv/h)	Immediately after dosage administration	3.92 ± 0.63 (2.69-5.61)	2.68-4.47	[60,61,72,73]
	Per unit administered activity (µSv/h/GBq)
	At the time of Discharge	7.05 ± 2.94b	7.0 ± 3.0c
			9.8 ± 3.1d
In-vivo Kinetics (Hours)	Fast component	2.23 ± 0.95e (0.82-4.90)	1.28-4.7	[64,66,72]
	Slow component	36.20 ± 16.47 (12.15-71.94)	49.5-87.2
Urinary Excretionsf (%)	0-2 Hours	35		[64,73]
	0-6 Hours	55	44-46
	0-24 Hours	70	58
Dosimetry Estimates [Absorbed dosef (mGy/MBq)]	Kidneys	0.72 ± 0.23	0.55-1.15	[65,74-81]
	Bone marrow	0.041 ± 0.021	0.03-0.07
	Total Body	0.071 ± 0.025	0.05-0.07
	Tumors	8.83 ± 7.18	3.41-9.7

^aDose rate at 1-meter distance from the surface of patient’s body.

^bDischarge time was 18-20 hours after dose administration.

^cDischarge time was 18 hours after dose administration.

^dDischarge time was 4-5 hours after dose administration.

^eDuring our study, we also found that the clearance of ¹⁷⁷Lu follows bi-exponential decay in 83.33% NET patients and in 16.67% NET patients it follows mono-exponential decay.

^fResults from ongoing ¹⁷⁷Lu-DOTATATE dosimetry work.

In vivo kinetics of lutetium and labelled radiopharmaceuticals

In vivo kinetics of lutetium

Till date there is no data available about the kinetics of lutetium in human body. However, data of lutetium kinetics in laboratory animals revealed that lutetium collects in tissue and organs (60% in bones, 2% in the liver and 0.5% in the kidneys). In addition, lutetium is reported to have biological half-life of 3500 days in bone and the liver and 10 days in the kidneys [62]. Hence most of the lutetium that enters the body accumulates in the bone over a period.

In-vivo kinetics of [¹⁷⁷Lu]Lu-DOTATATE

After administration of [¹⁷⁷Lu]Lu-DOTATATE intravenously to the NET patients, [¹⁷⁷Lu]Lu-DOTATATE uptake is seen in the kidney, liver, spleen and metastatic sites within 2 hours. Most of the [¹⁷⁷Lu]Lu-DOTATATE which in not absorbed in the body is quickly excreted out from the body in the urine. As per the literature reports [63], [¹⁷⁷Lu]Lu-DOTATATE is primarily excreted in the urine, with a cumulative excretion of 44% with in 5 hours, 58% within 24 hours and 65% within 48 hours. In our experience of 24 hours urinary excretion studies, we found a cumulative excretion of 35% within 2 hours, 55% within 6 hours and 70% within 24 hours (results from ongoing ¹⁷⁷Lu-DOTATATE dosimetry work). Sandstrom et al [64] studied in-vivo kinetics in patients administered with [¹⁷⁷Lu]Lu-DOTATATE and reported that the clearance of [¹⁷⁷Lu]Lu-DOTATATE is biphasic inside the NET patients. The effective half-life of fast component is 1.28 h (range 0.93-1.52 h) and slow component is 49.5 h (range 45.1-56.6 h). During our studies, we also found that the clearance of ¹⁷⁷Lu is biphasic inside the NET patients with effective half-life of fast component is 2.23+/-0.95 hours and slow component is 36.20+/-16.47 hours (Table 11) [66].

ALARA principle for critical organs

Bone marrow and kidneys are the critical organs in PRRT. Proximal tubular reabsorption of the [¹⁷⁷Lu]Lu-DOTATATE and subsequent retention results in high radiation exposure to the kidney. To reduce the high kidney retention of [¹⁷⁷Lu]Lu-DOTATATE, positively charged amino acids are co-infused to completely inhibit reabsorption of the [¹⁷⁷Lu]Lu-DOTATATE in the kidney. The co-administration of these amino acids reduces the kidney dose significantly in the range of 9% to 53% [67]. Renal absorbed dose is further reduced by up to 39% by extending the infusion time of the amino acids solution over 10 h, and up to 65% by extending the protection over 2 days after radiopeptide administration, thereby covering the renal excretion phase more efficiently [68,69].

Internal dosimetry

The absorbed dose estimation of normal organs and tumour/metastatic lesions provides the means for optimizing the dose administration of radio-peptide in PRRT, thereby full therapeutic window can be achieved with limited radiation-induced side effects to normal organs, particularly the kidney and bone marrow, which are considered as dose limiting organs in PRRT. The MIRD (Medical Internal Radiation Dose) scheme provides reference techniques for internal dosimetry. PRRT dose estimates in organs are generally calculated using the MIRD scheme, with the basic formula [70]:

Ď = Ã×S = A₀ × τ × S

Where Ď is the Mean absorbed dose to target organ from a uniformly distributed activity in source organ, Ã is the Cumulative activity in the source organ, S is the ‘S’ factor, it is the radiation absorbed dose per unit cumulative activity and τ is the residence time/disintegrations corresponding to the total number of decays occurring in the organ divided by A₀. For individualized patient dosimetry the values of S-factor rescaled according to the mass of the patient organ. Once the integral activities in the organs of interest are determined using mathematical models, absorbed doses are generally calculated using dedicated software programs such as OLINDA/EXM 1.0 or its newer version OLINDA 2.2.0. For obtaining the integral activities in the organs of interest input data include data from blood, urine, and whole-body scans at different time intervals sufficiently covering decay of fast and slow component of the radioisotope after PRRT. The planar images are useful to derive biokinetics over time, while SPECT and SPECT/CT fused images, although requiring more time to acquire, provide insight into organ-specific three-dimensional activity distribution. In our experience the radiation absorbed dose (mGy/MBq) to the kidney, spleen, liver, bone marrow, whole body and tumour are 0.69 ± 0.23, 0.93 ± 0.58, 0.13 ± 0.06, 0.04 ± 0.02, 0.06 ± 0.02 and 4.91 ± 3.73 respectively (Table 11, results from ongoing ¹⁷⁷Lu-DOTATATE dosimetry work), which are comparable with that reported in the literature [68].

Conclusion

Thus in this treatise, the multifaceted aspects of PRRT were discussed (encompassing the radiopharmaceutical aspects, clinical PRRT services, radiation safety and dosimetry involving [¹⁷⁷Lu]Lu-DOTATATE PRRT obtained from indigenous resources), with clinical practice points and lessons learnt over last 10 years from the experience of a large volume PRRT centre in India. The specific areas of clinical challenges such as combination regimen for FDG avid lesions and duo-PRRT for large sized tumors were emphasized. The role of alpha therapy with [²²⁵Ac]Ac-DOTATATE is evolving at this point and its place in the practice needs to be defined. The practical and unclear issues were discussed with examples and reference drawn from the published literature in each domain, to provide the readers an in-depth holistic overview of this subject.

Disclosure of conflict of interest

None.