If you want a method of quantification of NADPH (only), there a rapid and sensitive detection method of NAD, NADH, NADP & NADPH using Amplite™ Assay Kits (see attached document titled Dosage by Kit). In toxicology experiment you can measure the activity of microsomal enzymes associated with NAPDH like, plesae find a summary of preparation of microsomes and enzyme measurements NADPH-Cytochrome c reductase or NADPH-Cytochrome P-450 reductase (attached file entitled dosage NADPH-enzymes. detailled information are given in the publication of Chow & Cornish (attached).