Parabolic quantitative structure-activity relationships and photodynamic therapy: application of a three-compartment model with clearance to the in vivo quantitative structure-activity relationships of a congeneric series of pyropheophorbide derivatives used as photosensitizers for photodynamic therapy.

An open three-compartment pharmacokinetic model was applied to the in vivo quantitative structure-activity relationship (QSAR) data of a homologous series of pyropheophorbide photosensitizers for photodynamic therapy (PDT). The physical model was a lipid compartment sandwiched between two identical aqueous compartments. The first compartment was assumed to clear irreversibly at a rate K0. The measured octanol-water partition coefficients, P(i) (where i is the number of carbons in the alkyl chain) and the clearance rate K0 determined the clearance kinetics of the drugs. Solving the coupled differential equations of the three-compartment model produced clearance kinetics for each of the sensitizers in each of the compartments. The third compartment was found to contain the target of PDT. This series of compounds is quite lipophilic. Therefore these drugs are found mainly in the second compartment. The drug level in the third compartment represents a small fraction of the tissue level and is thus not accessible to direct measurement by extraction. The second compartment of the model accurately predicted the clearance from the serum of mice of the hexyl ether of pyropheophorbide a, one member of this series of compounds. The diffusion and clearance rate constants were those found by fitting the pharmacokinetics of the third compartment to the QSAR data. This result validated the magnitude and mechanistic significance of the rate constants used to model the QSAR data. The PDT response to dose theory was applied to the kinetic behavior of the target compartment drug concentration. This produced a pharmacokinetic-based function connecting PDT response to dose as a function of time postinjection. This mechanistic dose-response function was fitted to published, single time point QSAR data for the pheophorbides. As a result, the PDT target threshold dose together with the predicted QSAR as a function of time postinjection was found.

Design and construction of a light-delivery system for photodynamic therapy.

We have developed a device to divide the output from a dye laser into as many as eight beams of equal power with negligible total power loss. In this system, 630-nm s-plane polarized laser light was split by a series of highly polarization-sensitive plate beamsplitters. Each of the beams was coupled to a 200, 400, or 600 microm diameter optical fiber. Brewster-window-type attenuators allowed the power of each beam to be individually set. It was possible to reconfigure the device to produce four, two, or one output(s). We discuss the design requirements of the beamsplitter device and describe its construction from mostly commercially available components. An apparatus for positioning and stabilizing each optical fiber relative to the skin surface of a patient is also described. The illumination from the fiberoptic supported by such an apparatus strikes a defined surface area and is independent of patient movement. Both the beamsplitter device and the optical fiber positioner are used routinely in photodynamic therapy (PDT) of malignant tumors in the clinic and in the laboratory.

An in vivo quantitative structure-activity relationship for a congeneric series of pyropheophorbide derivatives as photosensitizers for photodynamic therapy.

An in vivo quantitative structure-activity relationship (QSAR) study was carried out on a congeneric series of pyropheophorbide photosensitizers to identify structural features critical for their antitumor activity in photodynamic therapy (PDT). The structural elements evaluated in this study include the length and shape (alkyl, alkenyl, cyclic, and secondary analogs) of the ether side chain. C3H mice, harboring the radiation-induced fibrosarcoma tumor model, were used to study three biological response endpoints: tumor growth delay, tumor cell lethality, and vascular perfusion. All three endpoints revealed highly similar QSAR patterns that constituted a function of the alkyl ether chain length and drug lipophilicity, which is defined as the log of the octanol:water partition coefficient (log P). When the illumination of tumor, tumor cells, or cutaneous vasculature occurred 24 h after sensitizer administration, activities were minimal with analogs of log P < or = 5, increased dramatically between log P of 5-6, and peaked between log P of 5.6-6.6. Activities declined gradually with higher log P. The lack of activity of the least-lipophilic analogs was explained in large part by their poor biodistribution characteristics, which yielded negligible tumor and plasma drug levels at the time of treatment with light. The progressively lower potencies of the most lipophilic analogs cannot be explained through the overall tumor and plasma pharmacokinetics of photosensitizer because tumor and plasma concentrations progressively increased with lipophilicity. When compensated for differences in tumor photosensitizer concentration, the 1-hexyl derivative (optimal lipophilicity) was 5-fold more potent than the 1-dodecyl derivative (more lipophilic) and 3-fold more potent than the 1-pentyl analog (less lipophilic), indicating that, in addition to the overall tumor pharmacokinetics, pharmacodynamic factors may influence PDT activity. Drug lipophilicity was highly predictive for photodynamic activity. QSAR modeling revealed that direct antitumor effects and vascular PDT effects may be governed by common mechanisms, and that the mere association of high levels of photosensitizer in the tumor tissue is not sufficient for optimal PDT efficiency.

Interstitial photoradiation therapy for primary solid tumors in pet cats and dogs.

Photoradiation therapy, a new method for treatment of solid malignant tumors, depends upon the tumor localization and retention of hematoporphyrin derivative, which is activated in vivo by light in the red region of the spectrum. As currently applied to cutaneous and s.c. lesions, the light dose is limited by both normal tissue reactions and the effective penetration of the light through the tissues. In this report, primary solid malignant lesions in pet cats and dogs have been treated by interstitial photoradiation therapy by applying the activating light from a laser [635 +/- 5 (S. D.) nm] directly into the tumor masses thrugh a 200-micrometer quartz fiber optic. Twelve of 14 lesions (four osteosarcomas, two squamous cell carcinomas, two malignant melanomas, one mast cell sarcoma, one fibrosarcoma, one sebaceous gland sarcoma, and a metastatic prostatic carcinoma) responded to treatment, and three are currently considered permanently controlled at 1 year or more following treatment. This method has not only allowed photoradiation therapy to be applied to some remote lesions but has also nearly eliminated normal tissue effects, thus greatly extending the applicability of this treatment to a wide range of human tumors.

Photoradiation in the treatment of recurrent breast carcinoma.

Photoradiation, with the use of hematoporphyrin derivative (Hpd) activated by visible light in the red region of the spectrum, was an effective treatment for controlling local and regional chest wall recurrences of breast carcinoma. With sufficient time between iv injection of the drug and local activation with red light, cutaneous and subcutaneous masses were treated effectively without undue damage to overlying and adjacent skin. This high therapeutic ratio resulted from the ability to Hpd to accumulate and/or to be retained to a higher degree in malignant tissue than in many normal tissues. This technique can be used as a primary treatment or upon tumor recurrence following conventional modalities such as surgery, chemotherapy, and radiation therapy.

Photoradiation therapy for the treatment of malignant tumors.

Administration of hematoporphyrin derivative i.v. followed by local exposure to red light has resulted in complete or partial response in 111 of 113 cutaneous or s.c. malignant lesions. Tumors treated have included carcinomas of the breast, colon, prostate, squamous cell, basal cell, and endometrium; malignant melanoma; mycosis fungoides; chondrosarcoma; and angiosarcoma. No type has been found to be unresponsive. In several cases complete clearing of chest wall metastatis has been achieved in treated areas. Deep-seated and pigmented tumors required a higher dose of drug for effective treatment than did the more superficial and nonpigmented lesions. A high therapeutic ratio between tumor and skin response has been obtained by allowing at least 3 days between drug injection and exposure to the therapeutic light for 2,5-mg/kg doses and at least a 4-day interval for 5.0-mg/kg doses.

Energetics and efficiency of photoinactivation of murine tumor cells containing hematoporphyrin.

Hematoporphryrin derivative at an intracellular concentration in TA-3 mouse mammary carcinoma cells of 0.6 or 0.9 mM required input of 3.0 X 3.6 X 10(9) quanta/cell of red light (620 nm) to achieve a 90% kill. At an intracellular concentration of 1.2 mM, this light requirement drops to 1.5 X 10(9)quanta/cell. The energy for this photodynamic process is about 100 times higher than that required for ionizing radiation to achieve the same level of kill for these cells. The quantum yield for singlet oxygen formation (the cytotoxic agent in most photodynamic processes) from hematoporphyrin derivative is 0.75 +/- 0.07 in ethanol but only 0.16 +/- 0.07 within TA-3 cells, indicating possible intracellular complexing and quenching.

Identification of singlet oxygen as the cytotoxic agent in photoinactivation of a murine tumor.

Singlet oxygen, a metastable state of normal triplet oxygen, has been identified as the cytotoxic agent that is probably responsible for in vitro inactivation of TA-3 mouse mammary carcinoma cells following incorporation of hematoporphyrin and exposure to red light. This photodynamic inactivation can be completely inhibited by intracellular 1,3-diphenylisobenzofuran. This very efficient singlet oxygen trap is not toxic to the cells nor does it absorb the light responsible for hematoporphyrin activation. We have found that the singlet oxygen-trapping product, o-dibenzoylbenzene, is formed nearly quantitatively intracellularly when both the furan and hematoporphyrin are present during illumination but not when only the furan is present during illumination. The protective effect against photodynamic inactivation of the TA-3 cells afforded by 1,3-diphenylisobenzofuran coupled with the nearly quantitative formation of the singlet oxygen-trapping product indicates that singlet oxygen is the probable agent responsible for toxicity in this system.

Photoradiation therapy. II. Cure of animal tumors with hematoporphyrin and light.

Exposure of mouse and rat tumors of various types to more than 600 nm light 24 or 48 hours after an injection of hematoporphyrin resulted in a substantial number of long-term cures. Since hematoporphyrin is preferentially retained in tumor tissue, selective tumor destruction could be obtained. Light penetration studies and the high efficiency of this technique indicated its applicability even to certain deep-seated human tumors.