Spatially localized ballistic two-photon excitation in scattering media

Fluorescence anisotropy imaging in drug discovery

Non-invasive measurement of drug-target engagement can provide critical insights in the molecular pharmacology of small molecule drugs. Fluorescence polarization/fluorescence anisotropy measurements are commonly employed in protein/cell screening assays. However, the expansion of such measurements to the in vivo setting has proven difficult until recently. With the advent of high-resolution fluorescence anisotropy microscopy it is now possible to perform kinetic measurements of intracellular drug distribution and target engagement in commonly used mouse models. In this review we discuss the background, current advances and future perspectives in intravital fluorescence anisotropy measurements to derive pharmacokinetic and pharmacodynamic measurements in single cells and whole organs.

3D-resolved investigation of the pH gradient in artificial skin constructs by means of fluorescence lifetime imaging

PURPOSE

The development of substitutes for the human skin, e.g., artificial skin constructs (ASCs), is of particular importance for pharmaceutical and dermatologic research because they represent economical test samples for the validation of new drugs. In this regard, it is essential for the skin substitutes to be reliable models of the genuine skin, i.e., to have similar morphology and functionality. Particularly important is the barrier function, i.e., the selective permeability of the skin, which is strongly related to the epidermal pH gradient. Because the pH significantly influences the permeation profile of ionizable drugs such as nonsteroidal anti-inflammatory drugs, it is of major importance to quantitatively measure the epidermal pH gradient of the ASC and compare it to that of genuine skin.

METHODS

Using three-dimensional fluorescence lifetime imaging combined with two-photon scanning microscopy, we measured with submicron resolution the three-dimensional pH gradient in the epidermis of ASCs stained with 2',7'-bis-(2-carboxyethyl)-5/6-carboxyfluorescein.

RESULTS

Similar to genuine skin, the surface of the artificial epidermis has an acidic character (pH 5.9), whereas in the deeper layers the pH increases up to 7.0. Moreover, the pH gradient differs in the cell interior (maximally 7.2) and in the intercellular matrix (maximally 6.6). Apart from the similitude of the pH distribution, the genuine and the artificial skin prove to have similar morphologies and to be characterized by similar distributions of the refractive index.

CONCLUSIONS

Artificial skin is a reliable model of genuine human skin, e.g., in permeability studies, because it is characterized by a similar pH gradient, a similar morphology, and a similar distribution of the refractive index to that of genuine skin.

Use of coherent control methods through scattering biological tissue to achieve functional imaging

We test whether coherent control methods based on ultrashort-pulse phase shaping can be applied when the laser light propagates through biological tissue. Our results demonstrate experimentally that the spectral-phase properties of shaped laser pulses optimized to achieve selective two-photon excitation survive as the laser pulses propagate through tissue. This observation is used to obtain functional images based on selective two-photon excitation of a pH-sensitive chromophore in a sample that is placed behind a slice of biological tissue. Our observation of coherent control through scattering tissue suggests possibilities in multiphoton-based imaging and photodynamic therapy.