The utilisation of macromolecules in the therapy of cancer and other diseases is becoming increasingly important. Recent advances in molecular biology and biotechnology have made it possible to improve targeting and design of cytotoxic agents, DNA complexes and other macromolecules for clinical applications. In many cases the targets of macromolecular therapeutics are intracellular. However, degradation of macromolecules in endocytic vesicles after uptake by endocytosis is a major intracellular barrier for the therapeutic application of macromolecules having intracellular targets of action. Photochemical internalisation (PCI) is a novel technology for the release of endocytosed macromolecules into the cytosol. The technology is based on the activation by light of photosensitizers located in endocytic vesicles to induce the release of macromolecules from the endocytic vesicles. Thereby, endocytosed molecules can be released to reach their target of action before being degraded in lysosomes. PCI has been shown to stimulate intracellular delivery of a large variety of macromolecules and other molecules that do not readily penetrate the plasma membrane, including type I ribosome-inactivating proteins (RIPs), DNA delivered as gene-encoding plasmids or by means of adenovirus or adeno-associated virus, peptide nucleic acids (PNAs) and chemotherapeutic agents such as bleomycin and in some cases doxorubicin. PCI of PNA may be of particular importance due to the low therapeutic efficacy of PNA in the absence of an efficient delivery technology and the 10-100-fold increased efficacy in combination with PCI. The efficacy and specificity of PCI of macromolecular therapeutics has been improved by combining the macromolecules with targeting moieties, such as the epidermal growth factor. In general, PCI can induce efficient light-directed delivery of macromolecules into the cytosol, indicating that it may have a variety of useful applications for site-specific drug delivery as for example in gene therapy, vaccination and cancer treatment.

Photochemical internalisation (PCI) is a unique intervention which involves the release of endocytosed macromolecules into the cytoplasmic matrix. PCI is based on the use of photosensitizers placed in endocytic vesicles that, following light activation, lead to rupture of the endocytic vesicles and the release of the macromolecules into the cytoplasmic matrix. This technology has been shown to improve the biological activity of a number of macromolecules that do not readily penetrate the plasma membrane, including type I ribosome-inactivating proteins (RIPs), gene-encoding plasmids, adenovirus and oligonucleotides and certain chemotherapeutics, such as bleomycin. This new intervention has also been found appealing for intracellular delivery of drugs incorporated into nanocarriers and for cancer vaccination. PCI is currently being evaluated in clinical trials. Data from the first-in-human phase I clinical trial as well as an update on the development of the PCI technology towards clinical practice is presented here.

Photochemical internalization (PCI) is a unique procedure for site‐specific delivery of several types of membrane‐impermeable molecules to the cytosol of target cells. The technology is based on photochemical‐induced release of endocytosed macromolecules from endosomes and lysosomes into the cytosol. The purpose of this study was to evaluate the therapeutic potential of PCI of the type I ribosomal‐inactivating protein gelonin in an animal model. The photosensitizer aluminum phthalocyanine disulfonate (AlPcS_2a) was injected intraperitoneally (10 mg/kg) into athymic female BALB/c (nu/nu) nude mice (8–9 mice per group) with subcutaneously growing human adenocarcinoma (WiDr) tumors 48 hr before exposure to 135 J/cm² of red light focused on the tumor. Six hours before light exposure a single dose of 50 μg gelonin was administrated intratumorally. Tumor growth was measured at least twice a week. After immunomagnetic separation of in vivo growing tumor cells the subcellular localization of the photosensitizer was evaluated by fluorescence microscopy. The photosensitizer localized in endocytic vesicles in in vivo growing WiDr cells. Furthermore, it was found that in vitro gelonin treatment of WiDr cells isolated from photosensitizer‐treated mice potentiated a light‐induced decrease of clonal survival. Complete remission in 6 of 9 (67%) of the treated mice were induced. Our findings indicate that photochemical treatment with the photosensitizer AlPcS_2a activates the cytotoxic potential of gelonin in vivo. These results demonstrate that the synergistic effect of combining photoactivation of photosensitizer located in endocytic vesicles and gelonin is indeed a result of PCI of gelonin.

Photochemical internalisation (PCI) is a novel technology for release of endocytosed macromolecules into the cytosol. The technology is based on the use of photosensitizers that locate in endocytic vesicles, and that upon activation by light induce a release of macromolecules from the endocytic vesicles. PCI has been shown to stimulate delivery of a large variety of macromolecules and other molecules that do not readily penetrate the plasma membrane. The preclinical evaluation of PCI has been performed with aluminum phthalocyanine disulfonate (AlPcS_2a) as photosensitizer. AlPcS_2a, due to its large number of isomers potentially with batch-to-batch ratio variations, is not an optimal photosenstizer for clinical use. Disulfonated tetraphenyl chlorin (TPCS_2a) has therefore been developed by di-imide reduction of disulfonated tetraphenyl porphine (TPPS_2a). The synthesized TPCS_2a contains 3 isomers as shown by HPLC with low (<4%) inter-batch variation with respect to isomer formation, less than 0.5% (w/w) of the starting material TPPS_2a and absorbs light at 652 nm. As prerequisites for a PCI photosensitizer TPCS_2a was found to localize in intracellular granules assumed to be endocytic vesicles. In cells in culture TPCS_2a-PCI induced activation of gelonin as seen by enhanced cytotoxicity, increased transfection efficacy by an enhanced green fluorescence protein (EGFP)-encoding plasmid, induced gene silencing by siRNA towards EGFP and induced in a synergistic manner tumor growth delay by TPCS_2a-mediated PCI of bleomycin in CT26.CL25 carcinomas growing subcutaneously in athymic mice. TPCS_2a-PCI of bleomycin was found superior to meso-tetraphenyl chlorin-based photodynamic therapy (mTHPC-PDT) with respect to inhibition of tumor growth. The tumor growth delay by PCI of bleomycin was independent of the time of bleomycin administration between 3 h prior to light to immediately after light, while bleomycin administered 24 h prior to or 24 h after the light exposure induced suboptimal or only additive effects on tumor growth delay respectively. TPCS_2a-PDT and -PCI induced indistinguishably strong edema the first 3–4 days after TPCS_2a-administration and only weak erythema the first day after TPCS_2a administration. In contrast, mTHPC-PDT induced moderate edema the first 7 days after mTHPC administration, but strong erythema resulting in open wounds and escar formation the first 2–3 days after mTHPC administration. The pharmacokinetic properties of TPCS_2a were evaluated in athymic mice. The plasma pharmacokinetics was best fit to a 2-compartment model with half-lives of 0.78 and 36 hrs. TPCS_2a was found to be a clinically suitable PCI photosensitizer for photochemical activation of molecules that do not readily penetrate the cellular plasma membrane.