We have developed a novel ADC platform using our proprietary linker, CL2A, which was designed with targeted delivery of SN-38 in mind. SN-38 is about 3 orders of magnitude more potent than irinotecan, its parent drug, but it cannot be administered systemically to patients because of its poor solubility and toxicity. The linker, CL2A, allows us to produce SN-38 conjugates that are soluble in water with excellent yields, as well as preservation of antibody binding and drug activity.
CL2A contains an antibody coupling group on one end and a chemical group on the other for binding with a drug. We have also added a short polyethylene glycol to improve the solubility of CL2A.
Furthermore, because SN-38 can be converted from its active lactone form to the inactive carboxylate form, by attaching CL2A close to the lactone ring at position number 20 of SN-38, the linker protects the lactone ring from opening up thereby maintaining the activity of SN-38. Another key feature of our ADC platform is that the linkage between CL2A and SN-38 is sensitive to both acidic and alkaline conditions and will allow the detachment of SN-38 at a rate of about 50% per day in vivo.
The final structure of our ADC is depicted below, with the pH-sensitive cleavable linkage highlighted. What differentiates our ADC platform from other companies is the high drug to antibody ratio of about 7 molecules of drug per antibody. That is to say, when our ADCs bind to their targets on cancer cells, they are delivering up to 7 molecules of SN-38 per antibody molecule into the blood or at the vicinity of the tumor, which may explain why our ADCs deliver more than 120-times the amount of SN-38 to the tumor when studied in an animal model, as compared to when irinotecan, the parent compound, is given. We can deliver this drug concentration because our drug is not supertoxic, thus permitting us to give higher antibody doses, in repeated therapy cycles, that we believe provide a better therapeutic index.