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Photodynamic therapy (PDT) is a method for the treatment of cancer that involves the administration of systemic or topical photosensitizing drugs that are preferentially taken up by the tumour and then activated in the presence of light to cause tissue destruction (Dougherty, 1988). Photodynamic therapy works by the generation of singlet oxygen that results in damage to cell membrane structures, microvascular ischaemia, and tissue necrosis. Numerous clinical trials of photodynamic therapy have been conducted over the past decade and PDT has been approved for clinical use in recurrent bladder carcinomas, obstructing oesophageal tumours, and early carcinomas of the bladder, oesophagus, stomach, and tracheobronchial tree. PDT has also been shown to provide curative treatment of early carcinomas of the head and neck, including the oral cavity, pharynx, and larynx (Biel, 1995;Feyh, 1996). Initially PDT was tested in advanced cancers of the head and neck that were untreatable or refractory to conventional therapy, however these trials produced only limited success (Schuller et al, 1984;Wile et al, 1984). A recent retrospective review of the clinical data available for the treatment of head and neck neoplasia using photodynamic therapy indicated that complete response rates of 89.5% are achievable for early squamous cell carcinoma of the head and neck (Biel, 1998). Based on a range of photosensitizers and treatment modalities, cure rates of 95% and 80% were obtained for carcinoma in situ and T1 squamous cell carcinoma of the vocal chord and oral cavity/tongue respectively (follow up 70 months) (Biel, 1998).Most of the available photosensitizers, until recently, have been mixtures of porphyrins such as haematoporphyrin derivative and Photofrin (Quadralogic Technologies, Vancouver, Canada). The main problem with these first-generation photosensitizers is that of prolonged skin photosensitivity. Phototoxic incidences of 20-40% have been reported during follow-up of patients having received Photofrin, with a mean duration of skin photosensitivity exceeding 6 weeks (Dougherty et al, 1990).The use of 5-aminolaevulinic acid (ALA) represents a different strategy in the administration of photosensitizers. ALA itself is not the photo-active drug, but rather it induces, in situ, the synthesis of a pure endogenous porphyrin called protoporphyrin IX (PpIX). The formation of PpIX forms part of the haem synthesis pathway and all nucleated cells that use oxidative metabolism are probably capable of forming this photosensitizer. However, malignant tissue appears to preferentially accumulate PpIX, forming the basis of photodynamic therapy in cancer (Battle, 1993;Kennedy and Pottier, 1994). Intravenous ALA is rapidly cleared from the body, with no PpIX fluorescence within the skin or other body organs detectable after 24 h (Kennedy et al, 1991).Because of this reduced phototoxicity and excellent tumour localizing properties, ALA has been used with great success for the treatment of several neoplastic diseases, particularly of the skin, b...

A number of experimental antibody mediated cancer therapies aim to redirect cytotoxic T cells (CTLs) of non-tumour specificity to cancer cells. It has been previously demonstrated that cancer cells targeted with recombinant HLAclass I/viral peptide complexes via antibody delivery systems can be killed by virus specific CTLs. This novel therapeutic system has been developed with a simple pre-clinical model using the recombinant anti-CD20 B9E9 scFvSA fusion protein to target HLA-A2/peptide complexes to CD20 ؉ve Daudi lymphoma cells. In vitro data confirmed that, although binding of the B9E9 scFvSA fusion protein alone to Daudi cells had no effect on their growth, effective CTL mediated killing of Daudi cells could be achieved by targeting with B9E9 sfvScSA and recombinant HLA-A2/MI complexes at dilutions as low as 100 pg/ml. In contrast the free HLA-A2/MI complexes only significantly inhibited CTL activity at concentrations in excess of 100 ng/ml. The production of a specific and effective anti-tumour CTL response is one of the central aims of cancer immunotherapy research. 1 Although this approach may develop as an effective therapy for some malignancies, difficulties with the specificity and level of expression of tumour peptides in tumour cells, 2 the frequency of down regulation of HLA class I expression in cancer cells 3 and the difficulty in upregulating the CTL response to these epitopes may limit clinical effectiveness.A number of alternative immunotherapy strategies have been developed that aim to use the effector functions of the cellular immune system but using CTLs of non-tumour specificity redirecting them to cancer cells via antibody based systems. These methods include bispecific antibodies, 4 antibody-superantigen fusion proteins 5 and antibody cytokine fusion proteins. 6 It has demonstrated previously that T cells of anti-viral specificity can kill cancer cells via antibody targeted delivery of recombinant MHC class I/peptide complexes used either as monomers 7 or as tetramers. 8 The B cell surface antigen CD20 serves as a model target for the delivery of HLA class I/peptide complexes. It is expressed on most B cell malignancies, has minimal internalization with bound antibody remaining immobilized on the cell surface for a number of days. 9 -11 The therapeutic use of monoclonal antibodies to CD20 has now become established as routine in B cell lymphoma. 12 As a further development from the use of intact monoclonal antibodies either in native form or radiolabeled, a number of recombinant antibody fragments have been developed. The recently described anti-CD20 tetravalent B9E9 scFvSA fusion protein is currently being investigated as a targeting system used with radiolabeled biotin in the radioimmunotherapy of lymphoma. 13 To study the ability of human CTLs of anti-viral specificity to interact with tumour cells targeted with HLA-A2/peptide complexes in a physiological setting, a pre-clinical model has been developed. Severe combined immunodeficient (SCID) mice are able to support functional human ...

The production of cytotoxic T cells with specificity for cancer cells is a rapidly evolving branch of cancer therapeutics. A variety of approaches aim to amplify anti-tumour cytotoxic T cell responses using purified peptides, tumour cell lysates or recombinant HLA/peptide complexes in differing antigen presenting systems. Using a two-step biotin-streptavidin antibody targeting system, recombinant HLA-class I/peptide complexes were attached to the surface of B cells via the anti-CD20 B9E9-scFvSA antibody-streptavidin fusion protein. Flow cytometry with a conformation dependant monoclonal antibody to HLA class I indicated that targeted HLA-class I/peptide complexes remain on the surface of B cells in culture for periods in excess of 72 h. PBMCs were stimulated in vitro for 8 -14 days using the autologous B cells as antigen presenting cells. Following a single cycle of stimulation specific cytotoxic T cell responses to targeted HLA-A2 complexes containing the M1, BMLF1 and Melan A peptides could be demonstrated by tetramer staining and Cr release assays. With the HLA-A2/BMLF1 complex up to 2.99% of CD8+ve cells were tetramer positive producing 20% lysis (E : T 10 : 1) of CIR-A2 target cells in an in vitro cytotoxicity assay compared to baseline levels of 0.09% tetramer +ve and 2% lysis in the unstimulated population. PBMCs from a healthy donor treated with two cycles of stimulations with targeted HLA-A2/Melan A complexes, demonstrated expansion of the melanA tetramer +ve population from 0.03% to 1.4% producing 15% lysis of Melan A pulsed target cells. With further consideration to the key variables of HLA/peptide complex density, the ratio of stimulator to effector cells and optimum cytokine support, this system should offer an easy and effective method for the in vitro amplification of specific cytotoxic T cell responses and warrants development for the in vivo induction of cytotoxic T cell responses in cancer therapy.

Acetaldehyde (AcH) produced in the physiological metabolism of ethanol can be potentially toxic and immunomodulating. The antitumour activity of a suicide gene system using adenovirus delivered alcohol dehydrogenase (ADH) to convert ethanol to acetaldehyde inside cancer cells has been investigated in vitro and in vivo. In vitro experiments confirmed the toxicity of acetaldehyde to a number of tumour cell lines. Daudi lymphoma cells grown in normal media increased by Day 4 to 650% of their starting number, while those exposed to 250 mM, 500 mM and 1 mM acetaldehyde reached 138, 30 and 5% respectively. Adenocarcinoma cells appeared to be less sensitive with CMT-64 cells and HeLa cells numbering 105 and 53% of their starting number by Day 4 with 1 mM acetaldehyde. After transduction with an adenovirus containing the human ADH beta 2 cDNA, CMT-64 cells exposed to 20 mM ethanol had a reduction in number to 74% by Day 2 and to 36% by Day 4. In a preclinical model with Ad-ADH CMT-64 cells, mice exposed to daily pulses of ethanol for 5 days formed tumours only 30% on Day 6 and 42% on Day 13 of the volume of those in mice exposed to water.The ability of this easily administered suicide gene system to produce significant effects on cell proliferation in vivo suggests that further optimized development is warranted.

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