Discussion
Here we report the first-in-human experience with
213Bi-DOTATOC TAT in NET patients refractory to nonradioactive octreotide and
90Y/
177Lu-DOTATOC. It was possible to image the biodistribution of the therapeutic compound with conventional 440 keV scintigraphy scans. Tumour binding was comparable to that in diagnostic SSR imaging with
68Ga-DOTATOC (Fig.
2). Therefore, we concluded that the in vivo stability and pharmacokinetics of
213Bi-DOTATOC are sufficient and comparable to those of DOTATOC labelled with other radionuclides. The clearance patterns of the time–activity curves obtained with ex-vivo gamma-counting of blood and urine samples were similar to previously reported values for intravenously administered diagnostic somatostatin analogues [
23,
24]. The time–activity blood values from venous blood samples following intraarterial injection should represent the amount of peptide that has passed the target organ and reached the systemic circulation and should represent a parallel shift to that seen in the time–activity curve following intravenously administration. However, we have previously observed that the relevance of the first-pass effect depends on the tumour burden in the liver and differences between patients are higher for the intraarterial administration route [
4]. Sufficient in vivo stability of DOTATOC labelled with the beta emitter has already been reported [
2,
10]. In vitro autoradiolysis of
213Bi-DOTATOC is prevented with ascorbic acid. However, in contrast to beta-DOTATOC, the alpha decay might translate into damage to the peptide. Nevertheless, once the alpha particle is emitted, the fate of the peptide is only of limited interest, as tumour targeting is only pivotal as long as the molecule is radioactive.
The reported patient cohort was rather heterogeneous, because this was not a clinical trial with prospectively defined inclusion criteria but represented the experience with surrogate treatments offered individually to patients in challenging situations. Due to the limited clinical experience with TAT the first patient was treated with a highly selective administration into the tumour feeding artery of a single rapidly progressive metastasis. In this first patient we were able to demonstrate that it is possible to overcome the resistance to beta treatment without significant acute toxicity. The next three patients were in a life-threatening situation when they received their first cycle of liver-selective TAT. In patient 3 an acute risk of vessel occlusion was resolved successfully (Fig.
4). In long term follow-up the progression-free survival was 36 months. This is a remarkable clinical result. In contrast, the 2-year overall survival of patients with undifferentiated pulmonary carcinoids refractory to etoposide/carboplatin is <20 % if treated with FOLFIRI or topotecan [
25]. Patient 4 was scheduled for liver-directed radionuclide therapy after an episode of hepatic encephalopathy. While the liver metastases demonstrated a positive response to the regionally intensified arterial therapy, the patient showed extrahepatic progression. However, due to improved liver function, the experimental TAT was stopped and he was able to receive chemotherapy again which was still effective for his bone metastases.
Patient 5 showed partial remission after the initial cycles of
90Y/
177Lu-DOTATOC, but then no further tumour shrinkage of the residual masses was achieved. Due to the favourable acute and mid-term toxicity observed in our first TAT patients, we individually offered TAT to this patient and achieved long-lasting complete remissions in both the primary tumour and liver metastases. Hepatic arterial infusion of TAT was indicated in patient 7 as disseminated liver involvement was considered the leading factor in the overall prognosis. Surprisingly, in addition to the expected liver response, we also found “side-efficacy” to various small bone marrow lesions. As the tumoral accumulation maximum (
U
max) of intravenously administered DOTATOC is about 70 min after injection (with 80 % of
U
max reached 45 min after injection), we did not anticipate sufficiently fast tumour targeting and did not expect to achieve a reliable tumour-to-blood dose ratio with a short half-life radionuclide such as
213Bi (45.6 min) [
24]. However, after we observed a response of bone marrow lesions in patient 7, we offered systemic TAT to patient 8 with disseminated bone marrow carcinosis in whom PRRT with beta particles was considered contraindicated. While treatment response in non-solid lesions is not evaluable according to RECIST criteria, we are able to report a remarkable low acute haematological toxicity. This is in accordance to the concept of cell selective micro-dosing due to the short path length (about two cell diameters) of an alpha particle, and consistent with the experience with CD33 targeted radionuclide therapy of AML. A
131I-labelled anti-CD33-mAb translated into a myeloablative bone marrow dose [
26]. In contrast, with
213Bi-labelled anti-CD33-mAb it was possible to induce a reduction of bone marrow blasts with tolerable haematological toxicity [
27].
We found a relevant decline in both TER and GFR in all our patients treated with TAT. In a study evaluating renal toxicity after
90Y-DOTATOC or
177Lu-DOTATATE therapy, of 23 patients who received a median dose of 12.2 GBq
90Y-DOTATOC, 12 showed a creatinine clearance loss of >5 % and 8 a loss of >10 %, and of 5 patients who received a median dose of 23.2 GBq
177Lu-DOTATATE, 3 showed a creatinine clearance loss of >5 % and 2 a loss of >10 % at 1 year. Based on these data, a kidney tolerance threshold of 28 Gy was recommended [
28]. In our patients the kidney tolerance dose had already been consumed during the extensive pretreatment with beta-emitting PRRT, and thus a higher grade of kidney impairment was expected. However, during the follow-up of 2 years, none of our patients developed severe kidney failure requiring dialysis. Nevertheless, none of our patients showed pathological renal test results when TAT was started. The toxicity of TAT could be even more severe in patients with renal risk factors or previously damaged kidneys, and therefore we still need to be cautious in the use of TAT in patients with a long life expectancy.
In a preclinical investigation
213Bi-DOTA-PESIN was compared with
177Lu-DOTA-PESIN [
29]. No tubular degeneration but rather glomerulopathy was observed with
177Lu-DOTA-PESIN. In contrast, with
213Bi-DOTA-PESIN, tubular degeneration was the leading pathology. Following glomerular filtration, radiolabelled peptides are reabsorbed and trapped in the tubular cells. Due to the short range of alpha particles the authors suggested that mainly tubular cells were irradiated. In contrast, beta particles may reach the radiosensitive glomeruli. Therefore, we monitored kidney function with dedicated
99mTc-MAG3 and
51Cr-EDTA tests for TER and GFR, respectively. With a 40 % decline in TER and a 30 % decline in GFR during a 2-year period in our patients the tubular impairment was only slightly higher than glomerular toxicity. This would support the assumption that some of the toxicity can be attributed to the preceding beta-PRRT. However, it is not legitimate to compare the kidney toxicity of first-line beta-PRRT with the toxicity of alpha-PRRT in pretreated patients and overlap with kidney toxicity from the earlier therapy. After
90Y-DOTATOC treatment kidney failure after a latency period of even 15 months has been reported previously [
30].
For a long time alpha emitters have been considered the perfect fit for Ehrlich’s “magic bullet” approach to cell-selective cancer therapy [
31]. The advantage of alpha over beta radiation in therapeutic nuclear medicine has already been demonstrated in preclinical studies [
6]. Octreotide analogues labelled with alpha particle emitters have also shown a reasonable therapeutic range in animals [
7,
8]. However, until now the limited availability of these radioisotopes has been a key challenge for the transfer to clinical use. To date, the required
225Ac to build a high-activity generator system to provide sufficient
213Bi for clinical use has only been possible by combining the maximum activity currently available from the three producers, i.e. Oak Ridge National Laboratory (Oak Ridge, TN), the Institute for Transuranium Elements (Karlsruhe, Germany) and the Institute for Physics and Power Engineering (Obninsk, Russia). However, the recent development of accelerator-driven processes that allow production of the mother nuclide for
225Ac/
213Bi generators in practically unlimited amounts may overcome these supply limitations in the near future [
32,
33].
Recently the use of
223Ra, another alpha-emitting radionuclide and an inherent bone seeker, has led to improved survival in patients suffering from bone metastases of prostate cancer [
34]. However, the stable linkage of divalent
223Ra(II) to antibodies or other carrier molecules has not been successful. We introduced the chelatable, alpha-emitting radionuclide
213Bi to clinical PRRT targeting the somatostatin receptors of NET patients. The chelate molecule DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) has already been linked to a variety of tumour-targeting molecules, and therefore our approach could be easily transferable to other targeted therapies in oncology.
In the first preclinical phase of our study, we had to develop suitable labelling conditions for
213Bi-DOTATOC. Finally, a reliable protocol for the synthesis of clinical doses of
213Bi-DOTATOC was successfully established (see
Materials and methods section). The
225Ac/
213Bi generator system developed at the Institute for Transuranium Elements was demonstrated to be able to reliably provide single doses of up to 2 GBq
213Bi-DOTATOC per injection. Labelling yields of
213Bi-DOTATOC reliably exceeded 99 %.
While the clinical application of alpha emitters promises advantages due to the short path length and the high linear energy transfer of the emitted particle (Fig.
1), there are also some challenges associated with the available nuclides. The main route (98 %) of
213Bi decay involves an initial beta decay with 435 keV mean energy immediately (delay 3.7 μs) followed by the therapeutic alpha decay to
209Pb which is a beta emitter with 198 keV mean energy (half-life 3.2 h) and decays to quasistable
209Bi (Supplementary Fig.
2). While the beta emissions contribute only about 8 % to the absorbed tissue dose [
35], the bremsstrahlung following the beta decay compromise the inherent imaging abilities of
213Bi that is based on the emission of 440 keV gamma ray with an emission probability of 26 % per decay. Due to the recoil of the alpha particle,
209Pb (3.2 h) is expected to leave the DOTA chelate and eventually even the target tissue to redistribute to nontarget organs, thus translating into high background noise.
The short half-life of
213Bi (45.6 min) is also challenging in regard to the limited temporal resolution of conventional scintigraphy. In addition, the spatial resolution of a gamma camera is insufficient to provide reliable microdosimetry. In a post-mortem autoradiography evaluation of a subject exposed to the alpha emitter
232Th, microscopic “hot spots” with doses up to 50-fold higher than those assuming a homogeneous organ biodistribution were found [
36]. Therefore the 440 keV gamma images of
213Bi-DOTATOC are not sufficient for imaging-based dosimetry. However, the value of pretherapeutic dosimetry is still a controversial debate for radioiodine therapy and even
124I-PET-based treatment regimens have not been shown to be superior to the use of standard activities [
37]. Therefore, dose finding in our patients was done in a similar empirical manner to the procedure used for the establishment of new chemotherapeutics with escalating activity fractions until dose-limiting toxicity was observed.
Another challenge for blood and bone marrow dosimetry is the redistribution of the unstable, beta-emitting decay product
209Pb, which might partially leave the tumour tissue. Free Pb is known to accumulate in erythrocytes which can translate to a perfusion-dependent increase in bone marrow radiation [
38]. However, the decay chain of clinically used
223Ra and
224Ra includes the formation of
211Pb (half-life 36.1 min) or
212Pb (half-life 10.6 h), which should show redistribution patterns similar to that of
209Pb, and is followed by an additional alpha emission in the succeeding decay schema. Nevertheless, they are not associated with remarkable bone marrow toxicity [
34,
38]. In contrast, the energy transfer from the beta radiation of
209Pb might be considered almost negligible, and also the long period to onset of anaemia observed in the follow-up of patients receiving TAT contradicts the thesis of directly related red blood cell or bone marrow toxicity. However, it would be of particular interest to develop dosimetry tools that might enable absorbed doses to be modelled and probabilities predicted for response and toxicity prior to TAT.
Haematological and kidney toxicity are the limiting factors in the use of
90Y/
177Lu-DOTATOC. With
213Bi-DOTATOC acute haematological toxicity was moderate even in a patient with disseminated bone marrow involvement (Fig.
7). This is in accordance with the observations of the ALSYMPCA trial. The haematological toxicity of the bone-seeking alpha emitter
223Ra was not significantly higher than that of placebo [
34]. In contrast, bone seekers labelled with a beta emitter can cause a relevant rate of grade-III thrombocytopenia and leucopenia [
39]. MDS/AML was observed in one patient 2 years after initiation of TAT. However, this patient was heavily pretreated with 16 GBq
90Y-DOTATOC and 24 GBq
177Lu-DOTATOC in multiple cycles starting 5 years before the onset of MDS. MDS/AML has been found in 4.2 % of patients treated with
90Y-Zevalin during long-term follow-up with a mean latency of 4.8 years [
40]. This is also in accordance with single case reports of MDS/AML from beta-PRRT centres in Rotterdam and Bonn [
3,
41]. Therefore, the MDS/AML that occurred in our patient matches well the typical latency period for a
90Y-related secondary malignancy.
With a follow-up period of 3 years, no myeloproliferative disorders were found in patients who had received the bone-seeking alpha emitter
223Ra [
34]. Nevertheless, alpha emitters produce a high rate of double-strand DNA breaks and might be associated with a higher number of secondary neoplasias. Therefore, thorough long-term follow-up will be pivotal once TAT is evaluated in prospective clinical trials for first-line or second-line therapy, especially in patients with a good prognosis. Our reported experience is not sufficient to conclude that alpha-emitting radiopharmaceuticals are superior to beta-emitting radiopharmaceuticals in general. For the reported patients, the duration of tumour control with the preceding
90Y/
177Lu-DOTATOC therapy was up to 3 years (Table
1), and the follow-up in patients receiving TAT was only 2 years at the time of this report. However, we successfully demonstrated the feasibility of overcoming beta resistance with alpha emitters as an additional treatment line.
Conclusion
All patients reported in this investigation were in different challenging situations when receiving TAT with 213Bi-DOTATOC. TAT was shown to be able to overcome resistance against beta radiation and resulted in a high number of long-lasting anti--tumour responses. 213Bi-DOTATOC was associated with moderate acute haematological toxicity, even in a patient with highly disseminated bone marrow involvement. Chronic kidney impairment remained in the acceptable range, and no other organ toxicity was observed. Bringing together the possibility of producing the alpha emitter 213Bi in sufficient quantities to support universal clinical application in combination with chelate-conjugated targeting molecules, our results with TAT might open new avenues in therapeutic nuclear medicine.