Potentiation of photodynamic therapy antitumor activity in mice by nitric oxide synthase inhibition is fluence rate dependent.

The effects of systemic administration of the nitric oxide synthase (NOS) inhibitor NG-nitro-L-arginine (L-NNA) in combination with photodynamic therapy (PDT) on tumor response, tumor oxygenation and tumor and normal skin perfusion were studied in C3H mice bearing subcutaneous radiation-induced fibrosarcoma tumors. Photodynamic therapy was carried out using the photosensitizer Photofrin (5 mg/kg) in conjunction with a low fluence rate (30 mW/cm2) and a high fluence rate (150 mW/cm2) protocol at a total fluence of 100 J/cm2. Low fluence rate PDT produced approximately 15% tumor cures, a response not significantly altered by administration of 20 mg/kg L-NNA either 5 min before or after PDT. In contrast, high fluence rate PDT produced no tumor cures by itself, but addition of L-NNA either pre- or post-PDT resulted in approximately 30% and approximately 10% tumor cures, respectively. The L-NNA by itself tended to decrease tumor pO2 levels and perfusion, but statistically significant differences were reached only at one time point (1 h) with one of the oxygenation parameters measured (% values < 2 mm Hg). Photodynamic therapy by itself decreased tumor oxygenation and perfusion more significantly. Addition of L-NNA before PDT further potentiated this effect. The L-NNA exerted its most striking effects on the PDT response of the normal skin microvasculature. Low fluence rate PDT caused severe and lasting shut-down of skin microvascular perfusion. With high fluence rate PDT, skin perfusion was initially decreased but recovered to persistent normal levels within 1 h of treatment. Administration of L-NNA reversed this response, converting it to complete and lasting vascular shut-down identical to that achieved with low fluence rate PDT. This effect was somewhat L-NNA dose dependent but was still marked at a dose of 1 mg/kg. It occurred whether L-NNA was given before or after PDT. The L-NNA did not alter the long-term vascular response of skin to low fluence rate PDT. The ability of L-NNA to correspondingly improve tumor response and severely limit skin vascular perfusion following high fluence rate PDT, while providing no benefit for the low fluence rate protocol, suggests that vascular changes in the tumor surrounding normal tissue contribute to the enhanced tumor curability with adjuvant L-NNA treatment.