Phenylalanine co-administration largely promoted DOX-induced increases in serum levels of BNP, CK, LDH, ALT and AST (p < 0.05), while BH4 supplementation or SXT treatment significantly inhibited the elevation of these markers in DOX-induced CHF rats (p < 0.001) (Fig. 6E). This evidence concerns the gene NPPB and congestive heart failure.