Antitumor Therapy Mediated by 5-Fluorocytosine and a Recombinant Fusion Protein Containing TSG-6 Hyaluronan Binding Domain and Yeast Cytosine Deaminase

Matrix attachment therapy (MAT) is an enzyme prodrug strategy that targets hyaluronan in the tumor extracellular matrix to deliver a prodrug converting enzyme near the tumor cells. A recombinant fusion protein containing the hyaluronan binding domain of TSG-6 (Link) and yeast cytosine deaminase (CD) with an N-terminal His(x6) tag was constructed to test MAT on the C26 colon adenocarcinoma in Balb/c mice that were given 5-fluorocytosine (5-FC) in the drinking water. LinkCD was expressed in Escherichia coli and purified by metal-chelation affinity chromatography. The purified LinkCD fusion protein exhibits a K-m of 0.33 mM and V-max of 15 mu M/min/mu g for the conversion of 5-FC to 5-fluorouracil (5-FU). The duration of the enzyme activity for LinkCD was longer than that of CD enzyme at 37 degrees C: the fusion protein retained 20% of its initial enzyme activity after 24 h, and 12% after 48 h. The LinkCD fusion protein can bind to a hyaluronan oligomer (12-mer) at a K-D of 55 mu M at pH 7.4 and a K-D of 5.32 mu M at pH 6.0 measured using surface plasmon resonance (SPR). To evaluate the antitumor effect of LinkCD/5-FC combination therapy in vivo, mice received intratumoral injections of LinkCD on days 11 and 14 after C26 tumor implantation and the drinking water containing 10 mg/mL of 5-FC starting on day 11. To examine if the Link domain by itself was able to reduce tumor growth, we included treatment groups that received LinkCD without 5-FC and Link-mtCD (a functional mutant that lacks cytosine deaminase activity) with 5-FC. Animals that received LinkCD/5-FC treatment showed significant tumor size reduction and increased survival compared to the CD/5-FC treatment group. Treatment groups that were unable to produce 5-FU had no effect on the tumor growth despite receiving the fusion protein that contained the Link domain. The results indicate that a treatment regime consisting of a fusion protein containing the Link domain, the active CD enzyme, and the prodrug 5-FC is sufficient to produce an antitumor effect. Thus, the LinkCD fusion protein is an alternative to antibody-directed prodrug enzyme therapy (ADEPT) approaches for cancer treatment.