Tayyaba Hasan, Ph.D.

Professor

Wellman Center for Photomedicine

Masaschusetts General Hospital

Research Interests

Photodynamic therapy (PDT) is increasingly used to treat a wide number of diseases. It involves the photoactivation of a light-responsive chemical, or photosensitizer, upon exposure to an appropriate wavelength of light.

Photoactivation initiates photochemical reactions that generate highly cytotoxic reactive oxygen species (ROS) that interact with proximal biological molecules (photosensitization). PDT exploits these interactions to bring about acute tissue necrosis to a localized area. Diseases, such as tumors, can be selectively targeted by the activating light, which is usually delivered by a laser device. The major aspects of PDT that are currently studied in our laboratory are:

(1) Site-directed PDT
(2) Mechanism-based PDT combination therapies
(3) Optical imaging

Site-directed PDT

To exert its acute cytotoxic effect, PDT entails two steps: (i) preferential localization of the photosensitizer, and (ii) spatial localization of the activating light. Photosensitizers generally accumulate in tumors more than in normal tissues, mainly due to the higher permeability of the tumor microvasculature. Combined with site-selective irradiation, the resulting dual selectivity minimizes normal tissue damage.

Vascular-targeted PDT
Besides directly targeting tumor tissues, PDT can be targeted to the tumor vasculature. This localization is an important factor affecting the treatment outcome of PDT. Following injection, most photosensitizers gradually extravasate from the vasculature into surrounding tissues, providing an opportunity for direct therapeutic intervention to either compartment. By using a small time interval between photosensitizer injection and light irradiation, it is possible to essentially confine the effects of PDT to the tumor vasculature. This causes vascular shutdown and tumor necrosis by lack of perfusion. We are currently investigating vascular-targeted PDT as a means to enhance the treatment outcome of pancreatic and prostate tumors with a focus on reducing tumor growth as well as the metastases associated with PDT.

Active tumor cell targeting
The rapid proliferation of tumor cells distinguishes them from normal cells. As a result, tumor cells express distinct cell-surface molecules at higher levels, providing opportunities to target these cells selectively. For the tumor cell-specific delivery of photosensitizers, our group is currently employing two strategies: photosensitizer immunoconjugates (PIC), and photosensitizer nanoparticle aptamer conjugate therapies (SNACT). The underlying principle forming the basis of this therapy is that targeted photosensitizer-conjugates selectively bind to specific tumor cells that express molecules recognized by the conjugates. Following internalization, the photosensitizers are released inside the tumor cells, whereupon light irradiation causes cell death. Photoimmunotherapy uses PIC to improve the specificity of photosensitizer delivery. Our group has developed a PIC by modifying a chimeric targeting antibody (C225) with a photosensitizer. Our PIC recognizes the extracellular domain of epidermal growth factor receptor (EGFR), which is overexpressed in many tumors. The application of photoimmunotherapy in our lab is directed towards the development of new treatments for ovarian cancer. SNACT involves the encapsulation of photosensitizers within nanoparticles coated in a targeting moiety (aptamers). Our group is studying the application of this technology to both ovarian and prostate cancers.

Targeting Microorganisms
The emergence of clinical isolates that are resistant to many or even all standard antimicrobial chemotherapeutics provides the necessary impetus to develop treatments that are not hindered by microbial resistant mutants.  Because of the acute nature of PDT killing, it is thought that microbes are unable to develop resistance to PDT. Indeed, no PDT-resistant microbe strains have been reported to date. We are developing microbial-specific photosensitizers for use in PDT that exploits the b-lactamase-producing phenotype of drug resistant pathogens.  Such microbe-targeted photosensitive drugs should greatly limit the degree of PDT damage to host tissues, and thus enhance treatment efficacies.

Mechanism-based PDT combination therapies
Our group is interested in the biological consequences of PDT at both the cellular and molecular level. Due to the high complexity of carcinomas and their ability to induce survival pathways and become resistant to chemotherapeutics, our lab is developing mechanism-based PDT combination treatments in which one treatment will nullify the tumor survival responses resulting from the other treatment.

Combining PDT with anti-angiogenesis agents
Vascular endothelium growth factor (VEGF) is a potent stimulant of new vascular growth. VEGF is an important factor in tumor growth, where high proliferation rates produce areas of hypoxia, which induces VEGF to improve perfusion. We have shown that PDT can increase VEGF expression. VEGF induction following sub-curative PDT can provide a stimulus to tumors and reduce the efficiency of this treatment, as oxygen-starved tissues may recover and tumor cells migrate into the blood stream (a process known as metastasis). Our group is currently studying treatments that inhibit the effects of VEGF for use in combination with PDT of both prostate and pancreatic cancers. We have found that VEGF-blocking agents reduce PDT-associated metastases to distant organs. Furthermore, we are developing VEGF contrast agents to monitor the increase in VEGF expression following PDT (see Optical Imaging for PDT Development). The ability to track VEGF expression following PDT gives us the opportunity to calculate precisely the dose of the VEGF counter-therapy, which will provide a patient-specific treatment to reduce possible side effects.

Combining PDT with cell-differentiating agents
ALA-PDT is an increasingly accepted treatment for skin cancers. It involves the conversion of 5-aminolevulinic acid (ALA) to protoporphyrin IX (PpIX), an active photosensitizer. One problem of ALA-PDT is its lack of tissue penetration, which can lead to a sub-curative PDT dose in deeper tissues, triggering cell survival responses that can affect the treatment outcome. Our lab has previously discovered that differentiating agents, such as Vitamin D, can increase photosensitizer levels after ALA administration, thereby enhancing the overall effect of ALA-PDT. In an orthotopic prostate tumor model, we found that methotrexate, a commonly prescribed differentiating agent, can nullify the increase in metastases associated with PDT. In collaboration with Dr. Edward Maytin at the Learner Research Institute in Cleveland, we are investigating the mechanisms behind PDT enhancement by differentiating agents and exploring the use of new compounds that can enhance PDT of skin tumors.

EGFR as a molecular target for PDT
Epidermal growth factor receptor (EGFR) overexpression is associated with the development of chemotherapeutic-resistant ovarian and other cancers. Although EGFR inhibition is an effective treatment modality for cancer, resistance to such agents is a serious problem. We have demonstrated that PDT combined with an immunotherapy that targets EGFR produces a synergistic reduction in tumor burden. Furthermore, in our animal studies, this mechanistically non-overlapping combination modality increased survival rates in orthotopic ovarian tumor models. This overall approach is being studied for other cancers, including pancreatic, prostate, and head and neck cancers.

Optical Imaging for PDT Development
Online imaging capabilities can provide a wide variety of structural, physiological, and molecular information in living systems. In order to understand and further optimize PDT effects, we are developing and utilizing various imaging technologies: (i) in vitro studies using a commercial confocal microscope, (ii) instrumentation of fluorescence microscopes for in vivo imaging of small animals and (iii) contrast agents for molecular imaging. We have imaged the intracellular localization of targeted and non-targeted photosensitizers using a commercial confocal microscope in order to understand the mechanisms of targeted PDT in vitro. For in vivo imaging of small animals, we have designed an intravital fluorescence microscope. The microscope has been used to monitor the delivery of the photosensitizer in the vascular and tissue compartments of tumors for PDT treatment planning, and to investigate the physiological changes in vascular permeability due to PDT as well as PDT-induced VEGF expression. We are also currently developing a fluorescence endoscope with high resolution down to the cellular level. In combination with a novel imaging agent providing high selectivity against ovarian cancer cells, we expect to enable ovarian cancer detection at earlier stages as compared to conventional diagnostic tools such as laparoscopes. Finally, we are developing contrast agents that will help in the understanding of the molecular effects of PDT. Based on the mechanistic investigation on PDT-induced VEGF, we have prepared a contrast agent and a molecular imaging strategy to monitor VEGF expression in vivo.