Pharmacokinetic data indicate hydralazine has an extensive and complex metabolism depending on acetylator status: slow acetylators undergo primary oxidative metabolism, while rapid acetylators are acetylated.
Mulrow JP, Crawford MH. Clinical pharmacokinetics and therapeutic use of hydralazine in congestive heart failure. Clin Pharmacokinet. 1989 Feb;16(2):86-9

Previous studies showed that several drugs inhibited quinine 3-hydroxylation, a cytochrome P450 (CYP) 3A4-mediated reaction, in vitro. In this extended study, 13 drugs were selected and tested by human liver microsomes in order to further determine their respective inhibition constant (Ki) and type of inhibition. According to the apparent Ki values, the inhibitory rank order of these tested drugs was as follows: ketoconazole > doxycycline > omeprazole > tetracycline > troleandomycin (with pre-incubation) > primaquine > troleandomycin (without pre-incubation) > nifedipine > erythromycin > verapamil > oleandomycin > diltiazem > cimetidine > hydralazine. Among these drugs, doxycycline, tetracycline, ketoconazole, nifedipine and hydralazine were judged as mixed inhibitors; whereas, the remaining other drugs tested were judged as competitive inhibitors. When the plasma/serum concentrations possibly attained after their usual therapeutic doses were taken into account, tetracycline, doxycycline, omeprazole, ketoconazole, nifedipine, troleandomycin and erythromycin are likely to be inhibitors of quinine metabolism in patients when these drugs are co-administrated with quinine.
Zhao XJ, Ishizaki T. A further interaction study of quinine with clinically important drugs by human liver microsomes: determinations of inhibition constant (Ki) and type of inhibition. Eur J Drug Metab Pharmacokinet. 1999 Jul-Sep;24(3):272-8.

An in vitro study reports a method using cryopreserved human hepatocytes and the time-dependent aldehyde oxidase (AO) inhibitor hydralazine (K(I) = 83 ± 27 μM, k(inact) = 0.063 ± 0.007 min(-1)), which estimates the contribution of AO metabolism relative to total hepatic clearance. Using zaleplon as a probe substrate and simultaneously monitoring the AO-catalyzed formation of oxozaleplon and the CYP3A-catalyzed formation of desethyzaleplon in the presence of a range of hydralazine concentrations, it was determined that >90% inhibition of the AO activity with minimal effect on the CYP3A activity could be achieved with 25 to 50 μM hydralazine. Differences in estimated f(m(AO)) between two batches of pooled human hepatocytes suggest that sensitivity to hydralazine varies slightly with hepatocyte preparations. Substrates with a CYP2D6 contribution to clearance were affected by hydralazine to a minor extent, because of weak inhibition of this enzyme.
Strelevitz TJ, Orozco CC, Obach RS. Hydralazine as a selective probe inactivator of aldehyde oxidase in human hepatocytes: estimation of the contribution of aldehyde oxidase to metabolic clearance. Drug Metab Dispos. 2012 Jul;40(7):1441-8.