Complex sentence: This may be compounded by the fact that dextromethorphan and chlorpheniramine are both metabolized by CYP2D6 isozyme of Cytochrome P450 which could increase the plasma concentration of both by inhibiting metabolism and increasing blood serum concentrations.
Split into simpler sentences: This may be compounded by the fact that dextromethorphan and chlorpheniramine are both metabolized by CYP2D6 isozyme of Cytochrome P450. This could increase the plasma concentration of both drugs by inhibiting metabolism and increasing blood serum concentrations.