The present study showed a rapid clearance of [177Lu]FAPI-46 from normal organs but relatively high accumulation in the PANC-1 tumour model. Dose-dependent tumour-suppressive effects were observed in both PANC-1 xenograft mice treated with [177Lu]FAPI-46 and [225Ac]FAPI-46, respectively. [177Lu]FAPI-46 showed slower but more prolonged therapeutic effects as compared to [225Ac]FAPI-46. We also performed [18F]FAPI-74 PET in PANC-1 xenograft mice and confirmed the high uptake in the tumour as well as the confirmation of FAP expression in the tumour stroma by immunohistochemistry.
We demonstrated the effectiveness of alpha therapy for FAP-expressing pancreatic cancer using [225Ac]FAPI-04 in a previous study [11]. [225Ac]FAPI-04 was thought to irradiate tumour cells by the alpha particles emitted from CAFs in the stoma. However, the alpha irradiation also has affects on CAFs, the primary site of accumulation, which are supporting tumour progression. Since beta particles have a more extended range in tissue compared to alpha particles, beta irradiation may reach tumour cells more homogeneously compared to alpha irradiation. Thus, we used FAPI-46 labelled with 177Lu, a beta emitter, for PANC-1 xenograft mice in the present study. Previous studies reported a rapid internalization of [177Lu]-labelled FAPI derivatives into HT-1080-FAP cells [4] and a high uptake in HT-1080-FAP tumour-bearing mice [4, 13]. In the present study, we also found a relatively high accumulation of [177Lu]FAPI-46 in PANC-1 xenografts, which is considered to target FAP mainly expressed in the stroma. In our previous study, [225Ac]FAPI-04 showed high accumulation in the liver [11], whereas the uptake of [177Lu]FAPI-46 in the liver was low in the experiments presented in this paper. A previous study also reported an increased accumulation of [225Ac]DOTATOC in the liver compared to [177Lu]DOTATOC [14]. The difference was thought to be due to the distribution of free 225Ac since a high uptake of released 225Ac in the liver was found in mice [15], suggesting better in vivo stability of [177Lu]FAPI-46.
In the present study, we found that [177Lu]FAPI-46 suppresses tumour growth in a dose-dependent manner. Meanwhile, other beta-emitters, such as 90Y, 188Re, and 153Sm, -labelled with FAPI derivatives were administered in humans without serious side effects [6, 10, 16], suggesting the potential clinical application of [177Lu]FAPI-46. Compared with [177Lu]FAPI-46, [225Ac]FAPI-46 showed faster therapeutic effects in PANC-1 xenograft mice with a shorter duration. The tumour size in mice treated with a high dose of [177Lu]FAPI-46 started to be reduced at 9 days after administration, with a slower growth compared to the control group. In contrast, the tumour growth was reduced immediately after administering a high dose of [225Ac]FAPI-46 while starting to regrow by day 12 with the same tumour growth speed as the control group. However, in a previous study, 225Ac showed a lower survival rate of cells compared to cells treated with 177Lu [17], according to more fatal double-strand breaks induced by alpha particles [18, 19]. Meanwhile, [225Ac]PSMA-617 was effective in metastatic prostate cancer patients refractory to [177Lu]PSMA-617 [20, 21]. We speculate that the reason for the difference seen in our study is due to the fact that the target cells of [177Lu]FAPI-46 and [225Ac]FAPI-46 were CAFs in the stroma as opposed to tumour cells. Stroma cells can tolerate a more fatal environment than other cells and are more radioresistant [22, 23]. However, the effects of alpha irradiation on tumour stromal cells remain to be clarified. Due to a heterogeneous distribution of the stroma and tumour cells causing a heterogeneous dose distribution it might be difficult for alpha particles to reach the tumour cells sufficiently. In contrast, the tumour cells are more likely to be irradiated by beta emission from [177Lu]FAPI-46.
The therapeutic effects of [177Lu]FAPI-46 and [225Ac]FAPI-46 were rather limited, with some of the tumour-suppressive effects being not significant compared to the control group. Since the clearance of FAPI in vivo is fast, the biological half-life of FAPI is short, but the physical half-life of 177Lu and 225Ac is relatively long. The unmatched half-life may be the reason for the limited therapeutic effects even with the use of FAPI-46 with improved retention. Radionuclides with a shorter half-life, such as 188Re (half-life = 17.0 h) or 211At (half-life = 7.2 h), maybe better for FAPI therapy by increasing the local dose. Although the procedure for labeling FAPI with 211At has not been established yet, [188Re]-labelled FAPI was synthesized successfully recently and administrated clinically [6]. Therapeutic effects of [188Re]- and [211At]-labelled FAPI should be compared in a future study to investigate these nuclides as more suitable option for FAPI treatment.
Renal toxicity was observed in patients treated with [177Lu]DOTATATE. Renal dysfunction might occur years after [177Lu]DOTATATE therapy, even under kidney protection [24, 25]. However, no histological change was observed in the kidneys after administering [177Lu]FAPI-46 and [225Ac]FAPI-46 in the present study. Although further evaluation should be performed in future studies, our results suggest the clinical feasibility of [177Lu]FAPI-46 and [225Ac]FAPI-46 treatment.
This study had several limitations. First, we used PANC-1 xenograft models with FAP expression in the stroma for the evaluation. However, stroma formation may be different from the tumour stroma in the patients. Therefore, patient-derived xenograft models may ensure in future work a better clinical translation. Second, the sample size of [225Ac]FAPI-46 was insufficient because of the limited supply of 225Ac. Third, we used radionuclides with a relatively long half-life to target therapy in the pancreatic cancer model. Evaluation of the therapeutic effects using shorter-half-life radionuclides should be further investigated in future studies.