More methemoglobin is produced by benzocaine treatment than lidocaine treatment in human in vitro systems

- Benzocaine produces extensive methemoglobin when exposed to blood and liver enzymes.
- Lidocaine produces no methemoglobin under the same conditions as benzocaine.
- Xylidine produces limited methemoglobin under the same conditions as benzocaine.

The clinical use of local anesthetic products to anesthetize mucous membranes has been associated with methemoglobinemia (MetHba), a serious condition in which the blood has reduced capacity to carry oxygen. An evaluation of spontaneous adverse event reporting of MetHba submitted to FDA through 2013 identified 375 reports associated with benzocaine and 16 reports associated with lidocaine. The current study was performed to determine the relative ability of benzocaine and lidocaine to produce methemoglobin (MetHb) in vitro. Incubation of 500 μM benzocaine with whole human blood and pooled human liver S9 over 5 h resulted in MetHb levels equaling 39.8 ± 1.2% of the total hemoglobin. No MetHb formation was detected for 500 μM lidocaine under the same conditions. Because liver S9 does not readily form lidocaine hydrolytic metabolites based on xylidine, a primary metabolic pathway, 500 μM xylidine was directly incubated with whole blood and S9. Under these conditions MetHb levels of 4.4 ± 0.4% were reached by 5 h. Studies with recombinant cytochrome P450 revealed benzocaine to be extensively metabolized by CYP 1A2, with 2B6, 2C19, 2D6, and 2E1 also having activity. We conclude that benzocaine produces much more MetHb in in vitro systems than lidocaine or xylidine and that benzocaine should be more likely to cause MetHba in vivo as well.