The Enhanced Permeability and Retention (EPR) effect is a highly variable phenomenon. To enhance EPR-mediated passive drug targeting to tumors, several different pharmacological and physical strategies have been evaluated over the years, including e.g. TNFα-treatment, vascular normalization, hyperthermia and radiotherapy. Here, we systematically investigated the impact of sonoporation, i.e. the combination of ultrasound (US) and microbubbles (MB), on the tumor accumulation and penetration of liposomes. Two different MB formulations were employed, and their ability to enhance liposome accumulation and penetration was evaluated in two different tumor models, which are both characterized by relatively low levels of EPR (i.e. highly cellular A431 epidermoid xenografts and highly stromal BxPC-3 pancreatic carcinoma xenografts). The liposomes were labeled with two different fluorophores, enabling in vivo computed tomography/fluorescence molecular tomography (CT-FMT) and ex vivo two-photon laser scanning microscopy (TPLSM). In both models, in spite of relatively high inter- and intra-individual variability, a trend towards improved liposome accumulation and penetration was observed. In treated tumors, liposome concentrations were up to twice as high as in untreated tumors, and sonoporation enhanced the ability of liposomes to extravasate out of the blood vessels into the tumor interstitium. These findings indicate that sonoporation may be a useful strategy for improving drug targeting to tumors with low EPR.

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Abstract:The photostability of the widely used autofluorescent proteins EGFP and EYFP and their fluorinated counterparts were compared by means of fluorescence correlation spectroscopy.We analyzed the reduction of the apparent diffusional time in analogy to fluorescence quenching in which the 'photon concentration' is treated as the quencher concentration.The quantum yields of photobleaching Φ bl of EYFP (6.1×10 -5 ) and EGFP (8.2×10 -5 ) are in agreement with the previously published values. Among the investigated mutants, EYFP has the highest photostability and there is an enhanced photobleaching in (2-F) Tyr-EYFP. It turns out that the chromophore fluorination has no significant influence on the photostability.

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