posted on 2021-09-30, 20:13authored byHuan Ma, Feize Li, Guohua Shen, Huawei Cai, Weihao Liu, Tu Lan, Yuanyou Yang, Jijun Yang, Jiali Liao, Ning Liu
As
an excellent target for cancer theranostics, fibroblast activation
protein (FAP) has become an attractive focus in cancer research. A
class of FAP inhibitors (FAPIs) with a N-(4-quinolinoyl)-Gly-(2-cyanopyrrolidine)
scaffold were developed, which displayed nanomolar affinity and high
selectivity. Compared with 90Y, 177Lu, 225Ac, and 188Re, 211At seems to be more
favored as a therapeutic candidate for FAPI tracers which have fast
washout and short retention in tumor sites. Thus, the current study
reported the synthesis of two FAPI precursors for 211At
and 131I labeling and the preliminary evaluation of 131I-labeled FAPI analogues for cancer theranostics. FAPI variants
with stannyl precursors were successfully synthesized and labeled
with 131I using a radioiododestannylation reaction. Two
radioactive tracers were obtained with high radiochemical purity over
99% and good radiochemical yields of 58.2 ± 1.78 and 59.5 ±
4.44% for 131I-FAPI-02 and 131I-FAPI-04, respectively.
Both tracers showed high specific binding to U87MG cells in comparison
with little binding to MCF-7 cells. Compared to 131I-FAPI-02, 131I-FAPI-04 exhibited higher affinity, more intracellular
uptake, and longer retention time in vitro. Biodistribution studies
revealed that both tracers were mainly excreted through the kidneys
as well as the hepatobiliary pathway due to their high lipophilicity.
In addition, higher accumulation, longer dwell time, and increased
tumor-to-organ ratios were achieved by 131I-FAPI-04, which
was clearly demonstrated by SPECT/CT imaging. Furthermore, intratumor
injection of 131I-FAPI-04 significantly suppressed the
tumor growth in U87MG xenograft mice without significant toxicity
observed. The above results implied that FAP-targeted alpha endoradiotherapy
(specific to 211At) should be used to treat tumors in the
near future, considering the chemical similarity between iodine and
astatine can ensure the labeling of the latter onto the designed FAPIs.