The effect of hyaluronic acid on the activity of intra-tumoral and intestinal β-Glucuronidases - a potential mechanism for increasing the therapeutic index of Irinotecan

3080 Introduction: Proprietary formulations containing the polysaccharide, hyaluronic acid (HA) utilise the unique physiochemical and biological properties of HA enabling it to function as an excipient imparting its own pharmacological activity through directing the entrained anticancer agent/s to over-expressed CD44 receptors. Once localized in the tumor, the HA/drug complex aggregates forming a drug depot, increasing drug retention at the site; subsequently efficacy is enhanced through increased co-internalisation of the drug and HA. In an attempt to overcome the dose-limiting toxicities of irinotecan (CPT-11), this drug was formulated with HA, resulting in a product called HyCAMP™. In an uncontrolled Phase l clinical trial of HyCAMP™ in metastatic colorectal patients the incidence of grade 3 or 4 diarrhea or neutropenia was less than has been observed in other studies. The median patient survival in this study was 16 months, compared with an expected ∼10 months based on historical controls. The objective of the current study was to elucidate the potential mechanisms by which HyCAMP™ may be imparting clinical benefit over CPT-11 alone. Methodology: The effect of HA on the key CPT-11 metabolic enzymes was determined by incubating varying concentrations of HA of 3 different MW (2, 10 & 860kDa) with the homogenates of i) liver, ii) spleen, iii) kidney, iv) large & small intestines and v) 8 colon cancer cell lines . Colorimetric assays to determine the effect of the HA on carboxylesterase (CE), β-glucuronidase (β-GLUC) and uridine diphosphate glucuronosyl transferase (UDPGT) were conducted on each tissue homogenate. Results: The HA component of the HyCAMP™ formulation did not alter the activity of CE or UDPGT but it did affect the activity of β-GLUC. In both the large and small intestine 2kDa HA was found to exert an inhibitory effect (70%) on the activity of β-GLUC whereas 10 and 860kDa did not. In the tumor cell line lysates physiological concentrations of both 10 and 860 kDa HA increased intra-tumoral β-glucuronidase activity (140%). Conclusions: HA may influence the activity of key enzymes involved in the metabolism of irinotecan, with different effects seen in different tissues. If these data translate in vivo , it is possible that the GI-tract toxicity of CPT-11 could be overcome by formulation with HA, where the 2kDa HA degradation products act as an inhibitor of β-GLUC, competing with SN-38G for the for β-GLUC active site resulting in less conversion of SN-38G to toxic SN-38. Conversely, in the tumor, the high MW HA upregulates β-GLUC activity and could result in a greater conversion of intra-tumoral SN-38G to the toxic metabolite SN-38, thereby increasing the efficacy of CPT-11. Together these two mechanisms could contribute towards increasing the therapeutic index of CPT-11when presented as the HyCAMP™ formulation.