Efficacy of ¹³¹I-generation 5.0 polyamidamine-mediated targeting peptide in the mice with medullary thyroid carcinoma

Objective: To synthesize the novel targeting probes ¹³¹I-generation 5.0 polyamidamine (PAMAM (G5.0))-SR (where SR: Ser-Arg-Glu-Ser-Pro-His-Pro), ¹³¹I-PAMAM (G5.0)-GP (where GP: Gly-Pro-Leu-Pro-Leu-Arg), and ¹³¹I-PAMAM (G5.0)-SR/GP (double targeting peptides) as experimental groups and evaluate their target capability toward medullary thyroid carcinoma (MTC). Methods: The target peptides SR, GP, and SR/GP were purified and analyzed by using high performance liquid chromatography and then covalently linked with the modified PAMAM (G5.0) to synthesize PAMAM (G5.0) peptides [PAMAM (G5.0)-SR, PAMAM (G5.0)-GP, and PAMAM (G5.0)-SR/GP]. The diameter and zeta potential of PAMAM (G5.0) alone and PAMAM (G5.0) peptides were detected by using dynamic light scattering. The modified PAMAM (G5.0) and PAMAM (G5.0) peptides were labeled with radioisotope ¹³¹I using the chloramine T method. The radiolabeling rate, radiochemistry purity, and stability were determined by using thin-layer chromatography. SPECT/CT imaging was performed in all groups at 4, 8, 12, 24, and 48 h after the probes were injected into the peritoneal cavity of the model mice. The ratio of target to non-target (T/NT) was then detected. The percentage injection dose per gram (%ID/g) with the tumor and important organs of the model mice were calculated. One way analysis was used to compare the T/NT in different groups at the same time, T/NT in same group at the different time, %ID/g at 24 h in different groups. LSD-t test was used to compare date between two groups. Results: The purity of purified PAMAM (G5.0)-SR, -GP, and-SR/GP were at 99%. The diameter of PAMAM (G5.0), PAMAM (G5.0)-SR, PAMAM (G5.0)-GP, PAMAM (G5.0)-SR/GP were 4.47, 5.70, 4.71, 5.95 nm, and zeta potentials were +37.95, +20.02, +28.34, +24.37 mV respectively. The radiolabeling rates of the four types of ¹³¹I probes were above 75%. The radiochemistry purities of the purified probes were more than 90% and remained over 85% after 48 h incubation at room temperature. All the T/NT of the experimental groups were higher than those of the control groups. The T/NT significantly increased in ¹³¹I-PAMAM (G5.0)-GP (6.03 ± 1.45) at 4 h (t = 3.235, P = 0.033; t = 3.843, P = 0.019) compared with those in the positive and negative control groups (2.18 ± 0.39 and 1.36 ± 0.00, respectively). The T/NT significantly increased in ¹³¹I-PAMAM (G5.0)-SR (5.12 ± 1.65, 4.82 ± 0.09, and 3.41 ± 1.01) at 8, 12, and 24 h (t = 4.004, P = 0.017; t = 3.388, P = 0.027; t = 4.180, P = 0.009, respectively) compared with that in the negative control group (1.50 ± 0.00, 1.43 ± 0.65, and 1.34 ± 0.81). The T/NT of ¹³¹I-PAMAM (G5.0)-SR (3.41 ± 1.01) was significantly higher than that of ¹³¹I-PAMAM (G5.0)-GP (2.10 ± 0.67) at 24 h post-injection (t = 3.990, P = 0.016). The tumor %ID/g in ¹³¹I-PAMAM (G5.0)-SR (1.80 ± 0.18) was higher than that in other groups at 24 h, but no significant differences were observed (F = 3.366, P = 0.059). The peak values of the T/NT and ID%g of the tumor were observed at 4 h in the ¹³¹I-PAMAM (G5.0)-GP and ¹³¹I-PAMAM (G5.0)-SR/GP groups and at 8 h in the ¹³¹I-PAMAM (G5.0) and ¹³¹I-PAMAM (G5.0)-SR groups. The T/NT value decreased to 57% from 4 h to 12 h in ¹³¹I-PAMAM (G5.0)-GP group. Conclusions The SR and GP peptides enhanced the target ability of ¹³¹I-PAMAMM (G5.0) on MTC cells and neovascularization. The ¹³¹I-PAMAM (G5.0)-GP probe may be suitable for diagnosis because of its rapid ingestion and excretion than other probes in the model mice. The ¹³¹I-PAMAM (G5.0)-SR probe may provide a new precision method for MTC treatment and follow-up because of its better target ability and longer residence time than other probes in the model mice. (authors)