68 Ga-FAPI PET only reveals neoplasia in a rat colorectal tumor model
Since the AOM/DSS could spontaneously induce inflammation-associated carcinogenesis in the colorectum, we established an AOM/DSS-induced rat colorectal tumor model to test whether FAPI-PET distinguishes malignancy from benign lesions. This model perfectly represents the pathological changes of human colorectal carcinomas (CRC) from normal-aberrant crypt foci–adenoma-carcinoma. The SUVmax of 68Ga-FAPI-04 and 18F-FDG PET were synchronously analyzed on rats administered with the AOM/DSS.
The FDG uptake increased in the colorectal region of all rats (n=8). The localization of the 18F-FDG was diffuse in the distant colon, proximal colon, and even intestine. In FAPI PET-positive cases, the localization of the 68Ga-FAPI was almost always in the distant colon. The SUVmax of 18F-FDG in the colorectal region varied from 1.6 to 4.8 in the fourth month, while the SUVmax of 68Ga-FAPI ranged from 0.3 to 4.5 (Fig. 3A). After the dissection, some of the rats were nodular hyperplasia and bowel wall invasion, and the others were only congestion and edema of the bowel (Fig. 3B).
Eight rats were divided into two groups, the neoplasia group and the inflammation group without neoplasia (Fig. 2B), which were verified by pathological examination (Fig. 3C). The expression of FAP and hexokinase2 (HK2) was detected by IHC in the neoplasia or the inflammatory tissue. HK2 is a key glycolysis enzyme. As shown in Fig. 3C, the HK2 expression was upregulated in both the inflammatory lesion and the neoplasia. In contrast, the FAP expression was increased merely in neoplasia. This observation was highly consistent with the double tracer imaging of FDG-PET and FAPI-PET (Fig. 3D). The SUVmax of 18F-FDG was 2.325 ± 0.64 and 2.250 ± 1.74 in the neoplasia group and in the inflammation group (p = 0.9380), while the SUVmax of 68Ga-FAPI-04 was 3.475 ± 1.07 in the neoplasia group and 0.675 ± 0.33 in the inflammation group (p = 0.0024), suggesting that 68Ga-FAPI PET imaging distinguishes malignancy from inflammation.
68 Ga-FAPI PET distinguishes malignancy from inflammatory lesions
Moreover, the 18F-FDG PET and 68Ga-FAPI-04 PET were performed on one rat with invasive neoplasia and polyp to avoid background interference. As shown in Fig. 3E, the invasive neoplasia was 18F-FDG-positive and 68Ga-FAPI-04 positive, while the polyp was only 18F-FDG positive. Two lesions were located in different parts of the colorectum (Fig. 3F). Consistent with the report, the AOM/DSS-induced tumors were more often in the distal colorectum . After dissection, the slices from two segments were analyzed by the autoradiography of 68Ga-FAPI-04 and the FAP staining. As shown in the lower panel of Fig. 3F, the radiation intensity of the distal slice was much higher than the proximal one. Consistent with the autoradiography, only the distal sections were FAP-positive. This malignant lesion was further confirmed by pathological examination (Fig. 3G).
68 Ga-FAPI PET signal increases along with colorectal cancer progression
To determine whether 68Ga-FAPI-04 PET can distinguish tumors from premalignant lesions, we synchronously analyzed the SUVmax of 68Ga-FAPI-04 and 18F-FDG PET at the indicated time points after AOM/DSS administration in a rat colorectal tumor model. After three months of administering AOM/DSS, 18F-FDG and 68Ga-FAPI-04 PET imaging (n=3) was synchronously performed monthly. As shown in Fig. 4A, the uptake of FDG was already high in the third month. The SUVmax of 18F-FDG was 3.5 ± 0.5 at the third month, while the SUVmax of 68Ga-FAPI-04 was only 0.227 ± 0.07. However, the 68Ga-FAPI-04 signal progressively increased since the third month. The SUVmax of 68Ga-FAPI-04 reached 1.867 ± 0.47 in the sixth month. However, FDG SUVmax was still 4.63 ± 1.06 at the same time point (Fig. 4B). The change fold of the SUVmax of 68Ga-FAPI-04 was 8.67 ± 2.68 at the sixth month compared with the third month (p < 0.05), while the change fold of the SUVmax of 18F-FDG was 1.32 ± 0.13. There were no significant changes in rat weight, which was 1.09 ± 0.14 (Fig. 4B). This observation further supported the above finding that non-malignant inflammation does not affect 68Ga-FAPI PET imaging, distinguishing neoplasia from inflammation.