Vasculature-based differential tumor uptake of radiolabeled Ultrasmall Mesoporous Silica Nanoparticles in breast cancers models

4 Objectives: Limited success of nanomedicine in the clinic can be attributed to heterogeneity in “enhanced permeability and retention” (EPR) effect across tumor types and patients which determines the delivery of therapeutic nanoparticles to tumors and the resulting impact on efficacy. While some tumors may be highly perfused by abundant leaky vasculature, others exhibit substantial barriers to drug delivery including high interstitial fluid pressure and poor vascular perfusion. The ability to observe these differences in patients could thus be a useful tool for predicting how a patient will respond to treatment. Herein, we report radiolabeled ultrasmall mesoporous silica nanoparticles (USMSN) with prolonged blood circulation that show differential in vivo tumor accumulation based on vasculature discrepancies between two breast tumor models. Methods: USMSNs were synthesized using a facile one pot synthesis method, by carefully tuning the ratio of silica precursor and capping agent (polyethylene glycol-silane), followed by thorough characterization. USMSNs were further conjugated with 1,4,7-triazacyclononane-1,4,7-trisacetic acid (NOTA) chelator for radiolabeling with 64Cu (t1/2: 12.7 h). 4T1 and MDA-MB-231 tumor-bearing mice were injected with 3.7 - 5.5 MBq of 64Cu-NOTA-USMSN via tail vein and PET scans were performed at different time-points post injection (p.i.), followed by ex vivo biodistribution study. In addition, confocal microscopy of CD31 stained 4T1 and MDA-MB-231 tumor slices collected 48 h post-injection of FITC-conjugated USMSN was performed to further visualize the differential accumulation of USMSN throughout tumor tissue in correlation with tissue vascular density. Results: Highly uniform USMSN (~ 9.89 ± 1.27 nm), with a high specific surface area of ~ 156.2 m2/g were successfully synthesized. Incubation with ~ 37 MBq of 64CuCl2 resulted in rapid, time-dependent radiolabeling to a maximum yield of ~80 % in 2 h. 64Cu-NOTA-USMSN showed enhanced evasion of the reticulo-endothelial system (RES) resulting in prolonged blood circulation with significant signal from the blood pool even at the last time-point of scan (from 37.4 ± 3.1 at 1h to 8.3 ± 1.5 %ID/g at 48 h p.i). Importantly, 64Cu-NOTA-USMSN showed a rapid and enhanced accumulation in the 4T1 tumor, attributed to the EPR effect (4.4 ± 0.3, , 10.6 ± 3.5, 15.5 ± 1.5 and 17.7 ± 1.3 %ID/g at 1, 6, 18 and 48 h p.i. respectively). In comparison, despite similar blood circulation and RES evasion profiles, 64Cu-NOTA-USMSN uptake in MDA-MB-231 tumor was significantly smaller, (< 5%ID/g at all time-points tested). Such differences in tumor accumulation were dictated by distinct differences in tumor microvasculature (microvessel density, vascular sizes and vascular endothelial areas), as demonstrated by confocal microscopy. Conclusions: We have systematically investigated the microvasculature effect on in vivo pharmacokinetics of 64Cu-NOTA-USMSN in two models of breast cancer aided by extensive in vivo PET imaging and ex vivo microscopy. Radiolabeled USMSN demonstrated prolonged blood circulation and enhanced evasion of the RES. For similar vascular densities, larger vascular endothelium in 4T1 tumor models permitted enhanced accumulation and retention of USMSN, compared to smaller vascular endothelium in MDA-MB-231. Our results indicate the potential use of 64Cu-NOTA-USMSN as nanoreporters in predicting tumor permeability and conceivably therapeutic responses, thereby allowing patient selection and stratification in the clinic.