3.7 Biodistribution in A375 and A549 tumor-bearing mice
As shown in Figure 4D, in the A375 xenograft model, the uptake values of89Zr-IP150 in tumors at 72, 120, and 168 h were 14.28±1.92, 17.13±3.74, and 20.29±0.74 ID%/g, respectively, which were greater than those in other normal tissues. Co-injection of the unlabeled precursor IP150 significantly reduced the tumor uptake of89Zr-IP150 at 168 h p.i. by ~53% (p <0.01) but had no impact on normal tissues. In addition, the uptake of 89Zr-IP150 in MUC18 (-) A549 tumors was comparable to that in normal tissues. These data suggest that the uptake of 89Zr-IP150 was indeed mediated by MUC18.
Discussion
A growing body of research highlights the fundamental role of MUC18 in various pathologic processes; for example, MUC18 promotes atherosclerosis[27], induces insulin resistance induced by obesity[28], and contributes to the aggregation of infected red blood cells and lymphocytes[29], and low expression of MUC18 also leads to insufficient blood flow in the interstitial vascular region[30]. The worldwide incidence of cutaneous melanoma has been increasing annually at a more rapid rate than that of any other type of cancer[31]. In contrast to the increasing incidence of skin melanoma, the incidence of mucosal melanoma has remained stable[32]. Early and accurate diagnosis of melanoma can greatly improve patient survival. In addition to the pathogenesis of each subtype of melanoma, the response to BRAF/MEK mutation-targeted small molecule and immune checkpoint therapy is different[17]; therefore, antibody-based drugs targeting overexpressed tumor-associated antigens, such as ADC, are a more universally effective treatment approach[33]. The dominant expression of MUC18 in melanomas (i.e., in approximately 70% of primary melanomas and 90% of lymph node metastases) makes this marker a potential candidate for identifying primary and metastatic melanomas; at present, the Class 1 biologic drug targeting MUC18 “ADC AMT-253 for injection”, which has been approved for clinical research in the treatment of advanced solid tumors. Moreover, we hypothesized that IP150 would serve as an ideal targeting vector for the delivery of RDC. Currently, there are reports of radionuclide probes targeting MUC18, but the antibodies used are mouse-anti-human mAbs[34], which may cause a human anti-mouse antibody response and a low signal-to-noise ratio (SNR).
In this preclinical study, we propose an imaging approach that combines the high sensitivity and SNR of PET imaging with the biomolecular specificity of an anti-MUC18 humanized mAb for melanoma treatment.89Zr-IP150 is created by a high labeling rate, superior radiochemical purity, and good stability. We used three different MUC18 (+) melanoma cell lines as well as a MUC18 (-) cell line to evaluate the specificity and targeting of the probe in vitro and separately constructed subcutaneous CDX xenograft tumor models for in vivo studies. In A375, SK-Mel-28, and HMVII tumor-bearing mice,89Zr-IP150 showed a favorable biodistribution in which it selectively accumulated in tumors, was quickly cleared from the blood, produced low background signals starting at 48 hours and persisted at 168 hours post injection at the tumor site with high sensitivity. Furthermore, the distribution of89Zr-IP150 was desirable, with less accumulation in normal organs except for the liver, where exogenous antibodies are cleared[35], which is desirable for PET imaging. Figure 2 and Figure 3 show that 89Zr-IP150 also has good specificity and targeting ability both in vivo and in vitro in a mucosal melanoma model (HMVII). Although the incidence of mucosal melanoma in the melanoma subtype is lower, the five-year survival rate is only 25% due to the lack of early and effective diagnosis, so our results may help these patients benefit from early and specific diagnosis to choose a better treatment plan. Because human melanomas consistently exhibit high glucose metabolism, 18F-FDG PET/CT imaging is particularly well suited for detecting these tumors[36]; however, we detected low radioactive signals of 18F-FDG in both the A375 and HMVII CDX models, possibly due to the inconsistency between human melanoma cell metabolism in immunodeficient mice and human melanoma metabolism.
89Zr-IP150 also showed significant uptake in MUC18 (+) PDX models of melanoma. We found that probe uptake in the PDX model continued for up to 96 hours. In addition, increased spleen uptake was also found in the PDX model.
These data may have implications for future clinical studies of ADC and CAR-T-cell administration or177Lu/90Y-labeled IP150 antibody-targeted therapy in patients. In addition, as an important tumor target, MUC18 is abnormally highly expressed in a variety of other tumors, and high expression of MUC18 leads to poor prognosis in a variety of tumors. In addition to these tumor types, studies have shown that reduced MUC18 expression plays a role in inhibiting tumorigenesis and carcinogenesis in colorectal cancer by inactivating the typical Wnt/β-catenin cascade[37].
Conclusions
In conclusion, this study introduced the preclinical application of a humanized radiolabeled MUC18-specific mAb probe, which was successfully employed to image various mouse models of humanized melanoma. Furthermore, the probe exhibited a promising targeting effect on tumors in a clinically representative melanoma PDX model. Importantly, the radiation dose in adults (0.065 mSv/MBq) was deemed relatively safe, supporting its potential for future clinical translation.