Rationale of Combined PDT and SDT Modalities for Treating Cancer Patients in Terminal Stage: The Proper Use of Photosensitizer

Abstract

We read with interest the recent article by Wang et al, reporting the clinical application of SF1 (Sonoflora 1)—a sensitizer that can be activated by light and ultrasound. SF1 is an analog of chlorophyll in that its macrocycle backbone is porphyrin-based and the center of the porphyrin ring consists of a metal ion. The chlorophyll derivative has light absorption peaks at 402 and 636 nm. A very unusual combination of this investigational agent and various prototype light sources and ultrasound devices was used in the treatment of three patients who suffered from late stages of breast cancer with systemic metastases to multiple organs. The combination of photosensitizer and light illumination is commonly known as photodynamic therapy (PDT) and the combination of drug or sonosensitizer and acoustic activation as sonodynamic therapy (SDT). In theory, the direct tumoricidal effects of these modalities are mediated by cytotoxic agents generated through photochemical or sonochemical reactions inside tumor tissue. The narrow gap between potential benefit and potential risk is determined by intrinsic cellular sensitivity and the sensitizer ratio between tumor and normal tissue. The synergistic effects of sensitizer and low-power ultrasound have been examined in many in vitro studies and to a lesser extent in in vivo models. Although the sonosensitization-induced cytotoxicity might be attributed to the sonochemical reaction, such as sensitizer-mediated energy transfer and free radical generation, data suggest that the mechanism of sonosensitization is probably not governed by a universal mechanism, but may be influenced by multiple factors including the nature of the biological model, sensitizer distribution, ultrasound parameters and acoustic energy distribution. Nevertheless, to date, the correlation between ultrasound parameters and biological effects has not been fully established in preclinical investigation (in vitro or in vivo). There is no convincing data that shows that ultrasound used in this way is effective in the treatment of primary tumor and multiple metastases. Therefore, this article raises a few basic questions. First of all, without those critical safety and efficacy information, it is unjustifiable to test the unproven prototypes in humans, particularly those in terminal stages for any purposes. The authors state that acoustic activation was delivered after intravenous administration of the chlorophyll derivative in two different modes—local and whole body ultrasound activation. A handheld ultrasound transducer was used for two patients, which was associated with a side effect of pain. Another patient was placed in a water tub to receive whole body ultrasound treatment. It was expected that ultrasound could reach deep-seated tumor(s) and sensitizers that are beyond the reach of external light of 630 nm. Although no details were provided in terms of the number of transducers in the water tub, the geometric distribution of acoustic energy within the body, and its accessibility to the multiple tumor sites (e.g., breast, lung, bones, liver, neck, axillary lymph nodes, and abdominal lymph nodes), the authors indicated that high acoustic power (2 W/cm at 1 MHz for 20 minutes) was used in both settings without providing justifications on the selection of those parameters. It is also questionable that sufficient acoustic energy will reach the deep-seated metastases in lung and bone tissues. Second, single high-dose PDT and metronomic PDT (i.e., to deliver photosensitizer and/or light doses at low rates over an extended period) have distinct and complementary therapeutic goals. In contrast to the conventional PDT regimen of single dose drug administration (intravenous or oral), the authors used a repeated lingual delivery of the chlorophyll derivative (a total dose of 30 or 60 mg) for 2 to 3 consecutive days followed by light plus ultrasound treatment daily for 3 days starting at day 3 or day 4 after the onset