Photodynamic Properties Of A Bispyrrolecarboxamide-Modified Hypocrellin B

Photodynamic therapy (PDT), a promising treatment modality for cancer and other nonmalignant conditions, utilizes reactive oxygen species (ROS), such as singlet oxygen (1O2), the superoixde anion radical (O2-.), and the hydroxy radical (.OH), generated from the interactions between photosensitizer, visible or near infrared light, biomolecules, and oxygen, to attack the desired targets. At the chemistry level, the possible targets of ROS include DNA, RNA, unsaturated lipids, and certain residues of proteins. Due to the short-lived character of ROS, it is expected that the close vicinity of photosensitizers to these targets will improve the bioavailability of ROS and thus the photodynamic activities of photosensitizers.As a new type of non-porphyrin photodynamic antitumor drugs, naturally occurring hypocrellins, including hypocrellin A (HA) and hypocrellin B (HB), present many advantages over hematoporphyrin derivative (HpD), such as single and definite chemical composition, ease in purification, low dark toxicity, dual sensitization mechanisms of Type I and Type II. In this work, HB was modified by bispyrrolecarboxamide group, which is structurally related to netropsin, a well known minor groove binder, and the relationship between the affinity to DNA and the photodamage to DNA of the resultant HB derivative (HB-Net) was investigated in depth. The main results are as follows.1. The UV-visible absorption spectra, the spin-trapping EPR measurements, and the DPA (9,10-diphenylanthracene) bleaching experiments showed that (1) HB-Net exhibits much improved light harvesting efficiency in the phototherapeutic window (600-900 nm) as the result of its remarkable absorption red shift with respect to HB; (2) HB-Net remains the generation abilities of semiquinone anion radical, superoxide anion radical, hydroxyl radical, and singlet oxygen, but with much lower efficiencies than that of HB, most likely due to its very short triplet excited state lifetime. 2. The DNA melting temperature measurements and EB (ethidium bromide) displacement experiments showed that HB-Net has strong affinity to dsDNA, especially to AT sequences. However, in either aerobic or anaerobic conditions, HB exhibits much higher photodamage ability to dsDNA than HB-Net, in line with the poor abilities of HB-Net to generate semiquinone anion radical and ROS. Once compared to 6-aminocarproic acid substituted HB (Cap-HB), the intermediate for constructing HB-Net, the positive role of the affinity to DNA emerges immediately in the DNA photodamage by HB-Net.3. The radical generation abilities of HB and HB-Net can be enhanced by ascorbic acid via the photoinduced electron transfer from ascorbic acid to the photosensitizers. Thus, in anaerobic condition and in the presence of ascorbic acid, HB-Net exhibited much higher photodamage ability to dsDNA than HB, due to its stronger affinity to dsDNA, which makes the radicals generated closer to dsDNA. The results suggest that the triad with the structure of electron donor-photosensitizer-biotarget binder may be a promising type of photosensitizer candidate, in which ascorbic acid is one of the choices for electron donors.