A dominant area of antibody research is the extension of the use of this mighty experimental and therapeutic tool for the specific detection of molecules for diagnostics, visualization, and activity blocking. Despite the ability to raise antibodies against different proteins, numerous applications of antibodies in basic research fields, clinical practice, and biotechnology are restricted to permeabilized cells or extracellular antigens, such as membrane or secreted proteins. With the exception of small groups of autoantibodies, natural antibodies to intracellular targets cannot be used within living cells. This excludes the scope of a major class of intracellular targets, including some infamous cancer-associated molecules. Some of these targets are still not druggable via small molecules because of large flat contact areas and the absence of deep hydrophobic pockets in which small molecules can insert and perturb their activity. Thus, the development of technologies for the targeted intracellular delivery of antibodies, their fragments, or antibody-like molecules is extremely important. Various strategies for intracellular targeting of antibodies via protein-transduction domains or their mimics, liposomes, polymer vesicles, and viral envelopes, are reviewed in this article. The pitfalls, challenges, and perspectives of these technologies are discussed.

IntroductionPhotodynamic therapy (PDT) is a relatively new cytotoxic treatment, predominantly used in anticancer approaches, that depends on the retention of photosensitizers (PS) in tumour cells and their activation within the tumour through irradiation with light of the appropriate wavelength. Photoactivated PS generate reactive oxygen species (singlet oxygen, 1 O 2 , and free radicals, such as ·OH, HO· 2 and ·O 2 -) which are able to damage cellular structures, meaning that PDT is particularly attractive as an alternative means to kill drug-and radioresistant tumour cells.1,2 Normal cells, however, are also able to accumulate PS and be damaged by them, so that prolonged skin photosensitization, light-sensitivity of the eye and other side-effects have proved to be severe limitations of PDT. When photoactivated, PS inflict damage on many types of biomolecules without any specificity, their action being mediated largely via the reactive oxygen species mentioned, none of which travel more than several tens of nanometers before reacting with a biomolecule. Keeping in mind that cell dimensions are of the order of micrometres or tens of micrometres, it is clear that the intracellular action of PS is restricted to the site of their subcellular location and the surrounding radius of not more than 40 nm.3-5 That uneven intracellular distribution of PS can lead to differences in toxicity has been shown using laser microbeam irradiation. 6In contrast to cell membranes and other cytoplasmic organelles, the cell nucleus 7-9 is known to be a very sensitive target for reactive oxygen species. In order to reduce the dose of PS administered to patients and hence minimize the harmful side-effects of PDT, new approaches have been devised to increase the effectiveness of tumour-cell killing through targeted delivery of PS to hypersensitive subcellular sites. These approaches are the focus of the present review. Subcellular distribution of photosensitizersInsomuch as most mammalian cells span tens of micrometres, it is clear that PS efficiency will depend not only on the relative distribution of PS between the tumour and surrounding tissues and between malignant and normal cells, but also on the intracellular distribution of the PS. Furthermore, membranes divide the interior of the eucaryotic cell into compartments that differ markedly in their sensitivity to reactions induced by PS-generated reactive oxygen species, in the ability of damaged molecules to be replaced/recycled and in the extent to which such damage affects cell viability and/or the capability of the cell to divide. It is noteworthy that preferential PS accumulation in tumours is itself not a guarantee of selective photoinduced tumour damage and successful PDT. In experiments on rats with gliosarcoma 9L, 10 for example, it has been found that despite a 13-fold higher accumulation of the PS Photofrin in the tumour compared with Summary Photodynamic therapy (PDT) is a novel treatment, used mainly for anticancer therapy, that depends on the retention of photosensitizer...

The ability of nanoparticles and macromolecules to passively accumulate in solid tumors and enhance therapeutic effects in comparison with conventional anticancer agents has resulted in the development of various multifunctional nanomedicines including liposomes, polymeric micelles, and magnetic nanoparticles. Further modifications of these nanoparticles have improved their characteristics in terms of tumor selectivity, circulation time in blood, enhanced uptake by cancer cells, and sensitivity to tumor microenvironment. These “smart” systems have enabled highly effective delivery of drugs, genes, shRNA, radioisotopes, and other therapeutic molecules. However, the resulting therapeutically relevant local concentrations of anticancer agents are often insufficient to cause tumor regression and complete elimination. Poor perfusion of inner regions of solid tumors as well as vascular barrier, high interstitial fluid pressure, and dense intercellular matrix are the main intratumoral barriers that impair drug delivery and impede uniform distribution of nanomedicines throughout a tumor. Here we review existing methods and approaches for improving tumoral uptake and distribution of nano-scaled therapeutic particles and macromolecules (i.e. nanomedicines). Briefly, these strategies include tuning physicochemical characteristics of nanomedicines, modulating physiological state of tumors with physical impacts or physiologically active agents, and active delivery of nanomedicines using cellular hitchhiking.