Photodynamic therapy using m-tetra(hydroxyphenyl) chlorin. An animal model.

OBJECTIVE

To evaluate the potent photosensitizer m-tetra (hydroxyphenyl) chlorin (m-THPC) by using rabbits with cottontail rabbit papillomavirus-induced tumors and the canine larynx as model systems.

DESIGN

Nonrandomized control trial.

SETTING

Division of ear, nose, and throat research at a tertiary care teaching hospital.

MATERIALS

Rabbits were used for relative retention ratio studies and tissue tolerance tests. Studies on the swelling of normal tissues in the larynx after photoactivation were done with canines.

INTERVENTION

Animals were injected with 0.3 mg/kg of m-THPC. At varying intervals, tissues were exposed to 652 nm of light.

OUTCOME MEASURES

Outcome measures consisted of four elements: (1) decay of plasma concentration over time, (2) interval to and duration of maximal ratio between drug concentration in normal tissue and tumor, (3) maximal permissible light exposure to normal tissue (skin and laryngeal mucosa) at an optimal interval, and (4) efficacy--number of tumors with partial and complete response.

RESULTS

The largest papilloma to skin ratio (10:1) occurred 4 to 8 days after drug injection. The rabbit skin damage threshold was 40 to 60 J/cm2 at 6 days. The canine laryngeal edema and erythema thresholds were 50 to 70 J. A 75% cure rate of papillomas was achieved with tumors that were less than 100 mm2 in area at light doses that ranged from 25 to 75 J/cm2.

CONCLUSIONS

m-THPC shows efficacy in treating papilloma virus-induced tumors. We present a protocol for rapid optimization of the factors required for tumor destruction with minimal normal tissue damage, thus permitting determination of an optimal therapeutic protocol for any photosensitizer.