What is the relative potency of normeperidine compared to meperidine? 
I'm looking for citations of the two values from the same source, so
that the comparison between the two compounds are more likely to be
valid.  Potency can be defined either in terms of their mu receptor
affinity, their analgesic potency, or their respiratory depression
potency.  As you probably know, normeperidine is a metabolite of
meperidine.

Hi doctorchou2-ga, and thanks for your question.

After several unsuccessful searches, I finally came up with a useful
reference.  It turns out that normeperidine is a nonopioid metabolite
of meperidine with half the analgesic potency.  It also,
unfortunately, has about 2-3 times the excitatory stimulous of
meperidine in the central nervous system, which is the primary source
of the drug's toxicity.

Here is the relevant passage and reference for the article I mentioned above:

"Meperidine is metabolized in the body by two different pathways. The
predominant pathway is hepatic carboxylesterase metabolism to
meperidinic acid, an inactive metabolite. The most clinically
significant pathway is N-demethylation by the hepatic cyto-chrome
P-450 system to normeperidine, a nonopioid active metabolite. This
metabolic process is carried out in the microsomal hydroxylation
enzyme system. Normeperidine can be metabolized to either
normeperidinic acid by carboxylesterase or by microsomal hydroxylation
to N-hydroxynormeperidine followed by renal elimination.[24?34] The
active nonopioid neurotoxic metabolite normeperidine has half the
analgesic potency of meperidine but two to three times the potency as
a central nervous system (CNS) excitatory agent.[28,35] An overlooked
clinical iatrogenic event is the propensity of normeperidine to
precipitate anxiety, hyperreflexia, myoclonus, seizures, and mood
changes within 24 hours.[36]"

http://scholar.google.com/url?sa=U&q=http://www.formulasearch.com/download/Meperidine_Review.pdf

Latta KS, Ginsberg B, Barkin RL. Meperidine: a critical review. Am J
Ther. 2002 Jan-Feb;9(1):53-68. Review.
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11782820&query_hl=2


Search terms:
Using Google Scholar:
receptor affinity meperidine normeperidine


I hope this information is useful.  Please request any clarification
prior to rating.

Best,

   -welte-ga

thanks