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A major problem in the treatment of cancer is the specific targeting of drugs to these abnormal cells. Ideally, such a drug should act over short distances to minimize damage to healthy cells and target subcellular compartments that have the highest sensitivity to the drug. We describe the novel approach of using modular recombinant transporters to target photosensitizers to the nucleus, where their action is most pronounced, of cancer cells overexpressing ErbB1 receptors. We have produced a new generation of the transporters consisting of (a) epidermal growth factor as the internalizable ligand module to ErbB1 receptors, (b) the optimized nuclear localization sequence of SV40 large T-antigen, (c) a translocation domain of diphtheria toxin as an endosomolytic module, and (d) the Escherichia coli hemoglobin-like protein HMP as a carrier module. The modules retained their functions within the transporter chimera: they showed highaffinity interactions with ErbB1 receptors and A/B-importin dimers and formed holes in lipid bilayers at endosomal pH. A photosensitizer conjugated with the transporter produced singlet oxygen and Á OH radicals similar to the free photosensitizer. Photosensitizers-transporter conjugates have >3,000 times greater efficacy than free photosensitizers for target cells and were not photocytotoxic at these concentrations for cells expressing a few ErbB1 receptors per cell, in contrast to free photosensitizers. The different modules of the transporters, which are highly expressed and easily purified to retain full activity of each of the modules, are interchangeable, meaning that they can be tailored for particular applications.

The review summarizes the chemistry of the third generation of photosensitizers, namely, the derivatives of natural bacteriochlorophyll a, for photodynamic treatment of cancer. The compounds of this class strongly absorb light at lambda=770-850 nm. This unique property opens new therapeutic opportunities due to deeper tissue penetration of light, thereby increasing the photodamage for tumor eradication. Analyzed are the modifications of bacteriochlorophyll a, that improve physico-chemical characteristics of compounds and enhance accumulation in tumors. Focusing on the delivery of photosensitizers to the tumor site and to specific intracellular compartments, we describe the conjugates of bacteriochlorophyll a, derivatives with carbohydrate and protein carriers. Boronated bacteriochlorins can be used in both photodynamic and boron neutron capture therapy.

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