This study demonstrated that the liposome-encapsulated doxorubicin was selectively delivered to the irradiated tumors by use of a phage display-derived peptide. This TL-Dox had a long circulation half life and enhanced doxorubicin deposition within the tumors such that it last for more than 20 hours post the intravenous administration of the drug when 2-fold or higher doxorubicin were detected within the irradiated tumors than those within the blood stream. The NIR imaging and single dose pharmacokinetic studies suggested that the selective accumulation of liposome-encapsulated doxorubicin was resulted from the peptide-targeted delivery instead of the passive dosing effect. The improved drug accumulation within tumors resulted in measurable increases in tumor cell apoptosis and necrosis, and reductions in cell proliferation, blood vessel functionality and tumor growth rates. The tumor-specific delivery was regulated by spatially and temporally delivered radiation. The untreated tumors did not accumulate the HVGGSSV-decorated liposomes. These data, in combination with previous works demonstrating the specificity of the HVGGSSV peptide in multiple tumor models [25
], suggests specificity of the tumor-targeted drug delivery can be modulated with the phage display-derived peptide that selectively binds to the irradiated tumors.
Radiation plays important roles in the clinical treatment of most cancer patients, and the biological effects extend beyond direct cytotoxicity. Radiation increases vessel permeability by inducing expression of several inflammatory molecules. This phenomenon has been explored for targeting drug delivery and enhanced drug release [23
]. Compared to traditional targeted drug delivery that utilizes molecular targets over-expressed within tumors [19
], the radiation-guided drug delivery provides possibilities to concurrently improve targeting selectivity and intratumoral drug deposition efficiency with the spatially and temporally controlled delivery of radiation. The HVGGSSV peptide was isolated from in vivo
screening of a phage-displayed peptide library against irradiated tumors, it demonstrated robust selectivity to the irradiated tumor across multiple cancer models [25
]. The peptide did not bind to irradiated or LPS-inflamed normal tissues, and showed basal level accumulation within untreated tumors. Recently, Tax-interacting protein 1 (TIP-1) was identified as the molecular target that enables the peptide binding within irradiated tumors [40
]. Radiation induces translocation of the predominantly intracellular TIP-1 protein onto the plasma membrane surface within cancer cells. Studies also showed that TIP-1 translocation on the cell surface is not associated with radiation-induced inflammation. These data suggested that radiation-guided drug delivery by use of the HVGGSSV peptide would have minimal off-target effects in the radiation tract unlike targeting the radiation-induced inflammatory molecules.
Most of the conventional chemotherapeutic agents lack selectivity to cancer cells, the poor bioavailability within tumor and non-specific distribution of the drug throughout the whole body introduced systemic toxicity and limited effective treatment of the patients [2
]. Drugs encapsulated within nano-sized vehicles can be passively accumulated within tumor through the leaky tumor, by prolonged circulation time and the Enhanced Permeability and Retention effect [6
]. However, tumor-limited drug deposition and retention is more efficient by active targeting with tumor-specific antibodies or peptides [4
]. As evidenced with immunohistochemistry studies on the tumor tissues, the improved drug deposition within the irradiated tumors by the targeting liposomes resulted in increased cell apoptosis and decreased cell proliferation. Direct cytotoxic effects of doxorubicin cause cell apoptosis and decrease proliferation rate. Additionally, the dramatic reduction of the functional blood vessels within the IR plus TL-Dox treated tumors may also increase the number of apoptotic cells. Over the whole treatment course, the IR plus TL-Dox showed the best inhibitory effect on the tumor growth.
In contrast to the significantly elevated cell apoptosis, necrosis and reduced blood vessel functionality, TL-Dox and IR treatment only moderately affected tumor growth as compared to other treatment groups. The modest tumor growth control with the radiation-guided and tumor-targeted drug delivery may be attributed to the tumor models utilized in this study. The high sensitivity of the LLC and H460 tumor models to radiation alone hindered the ability to signify the full therapeutic potential of the radiation-guided tumor-targeted drug delivery. Also, the collapse of the tumor-associated blood vessels following the first drug dose could hinder subsequent drug delivery. We envision that more significant tumor growth control effects will be observed on radioresistant tumor models. Likewise, to better illustrate the therapeutic efficacy, a single administration of a high dose of the TL-Dox would be desirable to avoid effect of vessel collapse on the subsequent drug delivery [42
]. Alternatively, lower IR doses can be delivered to induce the HVGGSSV peptide binding by concurrent treatment of the tumor with tyrosine kinase inhibitors (TKIs) [25
]. The lower IR dose will highlight the therapeutic efficacy of the radiation-guided drug delivery. In fact, combination therapies with synergistic effects, such as the HVGGSSV drug delivery and TKIs, are desirable in delivering treatment to tumors. Therefore, in future studies we will investigate the effects of concurrent TKI therapy and alternative dosing schedules in a radioresistant model.
In summary, this study demonstrated that the phage display-derived hexapeptide HVGGSSV enabled radiation-guided selective drug delivery to tumors. Unique specificity of the peptide as illustrated within multiple tumor models suggests promising potentials of the peptide in tumor-targeted drug delivery.