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Frequently Asked Questions about the Erythromycin Breath Test |
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1. What does the Erythromycin Breath Test measure?
The Erythromycin Breath Test measures liver CYP3A4 catalytic
activity. CYP3A4 represents the major metabolic pathway
for therapeutically administered drugs. The test provides
a phenotypic measurement of individual variance in enzymatic
activity due to factors such as disease, diet and concomitant
medications as well as genetic differences.
2. How might the Erythromycin Breath Test be useful
in drug development?
• Identification of a New
Chemical Entity as a CYP3A4 inducer at therapeutic plasma
levels
Treatment with an identified inducer is expected to result
in accelerated clearance of many drugs metabolized by
CYP3A4. Such a response has been found with immunosuppressants
cyclosporin A and FK506, and for synthetic estrogens used
in birth control pills (1). This may result in therapeutic
failure of many drugs used in combination therapies. In
addition, it may be unwise to rely on contraceptive agents
with synthetic estrogens in clinical trials with a demonstrated
CYP3A4 inducer (1). The erythromycin breath test can also
be used to document that a patient’s CYP3A4 activity
has returned to normal prior to discharge from the study.
• Identification
of a New Chemical Entity (NCE) as an inhibitor of CYP3A4
at therapeutic plasma levels
Available data suggest that the Erythromycin Breath Test
not only identifies NCEs that inhibit CYP3A4, but also
can be used to estimate Ki in vivo for inhibition. For
example, the in vivo Ki for ketoconazole inhibition of
CYP3A4 was estimated as 1-2 µM (plasma concentration)
in a recent clinical trial employing the erythromycin
breath test (2). Thus, it is possible to rank order compounds
by their ability to inhibit CYP3A4 activity in vivo. A
NCE determined to be a potent inhibitor of CYP3A4 in vivo,
suggests that the NCE may produce potentially serious
drug interactions. For example, potentially serious drug
interactions resulting from CYP3A4 inhibition occur with
terfenadine and cyclosporin A (3,4). As with inducers,
the Erythromycin Breath Test can document that patient’s
CYP3A4 activity has returned to normal.
•
Determine whether CYP3A4 is rate
limiting in the in vivo elimination of a New Chemical
Entity
There is little correlation between in vitro and in vivo
drug kinetic predictions. In vitro systems can suggest
involvement of CYP3A4 in the metabolism of an NCE but
can’t show that liver CYP3A4 activity is rate limiting
in the elimination of the drug in vivo. Combining in vitro
data with the erythromycin breath test can support hypotheses
that CYP3A4 is rate limiting for NCE kinetics. If the
Erythromycin Breath Test significantly predicts the variation
in clearance of parent compound, it is reasonable to assume
that the elimination of the NCE will increase when patients
are treated with CYP3A4 inducers (ex. rifampin, some antiseizure
drugs and steroids) and will tend to decrease in patients
treated with CYP3A4 inhibitors (ex. some imidazole antimycotic
drugs and macrolide antibiotics).
• Evaluate
and develop New Chemical Entity’s that have narrow
therapeutic indexes
If CYP3A4 activity is rate limiting in the elimination
of a NCE, a wide variation in the kinetics of the NCE
in patient populations can be anticipated. As a result,
development of many CYP3A4 substrates has only been possible
when the therapeutic index is very wide, or when the NCE
is hoped to occupy a unique niche where blood level monitoring
is acceptable (ex. cyclosporin A, FK506). The erythromycin
breath test may be useful in guiding safe dosing regimens
of such a NCE by stratifying patients according to CYP3A4
activity.
• Assist
regulatory reviewers with complex drug pharmacokinetics
and potential drug interactions
Data obtained with the erythromycin breath test in clinical
trials can complement data obtained from in vitro systems
regarding metabolic pathways and drug interactions. Such
data may be used to justify performing fewer drug interaction
studies in vivo. For example, the absence of an effect
of a NCE on the Erythromycin Breath Test results together
with appropriate in vitro data may indicate that interaction
studies with CYP3A4 substrates (such as terfenadine, cisapride,
or hismanil) are not necessary.
• Utilize
the ERMBT to justify smaller Phase III clinical trials
The Erythromycin Breath Test can be used to identify low
or high "metabolizers" for intentional inclusion
in clinical trials. The erythromycin breath test can guarantee
that a study population includes individuals at the extremes
of CYP3A4 activity, regardless of the absolute size of
the study population.
3. What are the advantages of the erythromycin breath
test compared to measuring clearance of other CYP3A4
substrates?
There is now wide agreement among clinical pharmacologists
that the Erythromycin Breath Test provides an estimate
of liver CYP3A4 activity suitable for intra- and interpatient
comparisons. The advantages of the ERMBT over other probe-based
tests are:
•
Ease of use - The test can be performed in twenty
minutes, and the Erythromycin Breath Test includes analysis
and interpretation.
•
Use of a trace dose - The Erythromycin Breath Test
utilizes a trace dose of erythromycin (less than 0.1 micromole).
There are no pharmacological effects or fear of interaction
with other CYP3A4 substrates. The test can be added to
any study without fear of influencing CYP3A4 activity
towards other substrates (including other probes).
• Instantaneous
measure of activity - Because the conversion of
formaldehyde (produced from the demethylation of erythromycin)
to breath CO2 is very rapid, the rate of production of
label in the breath reflects the CYP3A4 activity at that
moment. When evaluating inhibitors, it is possible to
directly correlate plasma concentration (or unbound concentration)
directly with the percent fall from baseline in Erythromycin
Breath Test results, thereby making it possible to directly
calculate the in vivo IC50 for your inhibitor. This can
clarify the relationship between plasma concentration
and concentration of drug at the enzyme active site in
vivo, allowing correlation of metabolism obtained in vitro
systems.
•
Experience with the test - A critical question
when evaluating an inducer or inhibitor of CYP3A4 is the
clinical relevance of the magnitude of the effect observed.
The wide use of the Erythromycin Breath Test has created
a growing database for comparison of the effects observed
with a NME.
• Standardized Procedure
- Metabolic Solutions has standardized and validated the
entire test process to insure the uniformity of each Erythromycin
Breath Test. Kit manufacturing is conducted under current
Good Manufacturing Practices and breath specimen analysis
under current Good Laboratory Practices.
4. What is the radioactivity exposure involved with
the test?
Dosimetry calculations have been estimated from animal
data. The estimated exposure is 2 mrem, which corresponds
to about one quarter of one chest xray. Concern regarding
the theoretical health risks from such low level exposure
is diminishing as evidenced by the FDA approval for general
use a carbon-14 breath test for detection of Helicobacter
pylori, the organism responsible for gastric ulcers.
5. What is needed to be able to add the test to a
clinical protocol?
A site performing the erythromycin breath test must have
a license to receive and use radioisotopes. In studies
involving NMEs under an existing IND, an amendment to
the IND which cross-references Metabolic Solutions FDA
Drug Master File, is filed with the FDA. For non-IND studies,
an Institutional Review Board and a Radioactive Drug Research
Committee for human use of radioisotopes must approve
all protocols. The latter committee exists at most major
medical centers and serves as an extension of the Institutional
Review Board. We are unaware of any instance where approval
to use the test has been denied, and several major contract
research organizations now offer the test.
6. Are there CRO’s that can perform the Erythromycin
Breath Test?
The erythromycin breath test is a simple protocol that
can be performed at any CRO that has a license to handle
radiopharmaceuticals. There are many qualified CRO’s
in the United States and Europe.
References
1. Guengerich FP. (1990) Inhibition of oral contraceptive
steroid-metabolizing enzymes by steroids and drugs. Am.
J. Obstet. Gynecol. 163:2159-2163.
2. Watkins, PB, Jamis-Dow C, Collins J, Pearl M, Blake
D, Klecker RW. (1995) Assessment of in-vitro in-vivo drug
interaction predictions: Studies with taxol. Presented
at the AAPS meeting in Miami, FL.
3. Yun C-H, Okerholm RA, Guengerich FP. (1993) Oxidation
of the antihistaminic drug terfenadine in human liver
microsomes. Role of cytochrome P-450 3A(4) in N-dealkylation
and C-hydroxylation. Drug Metab. Dispos. 21:403-409.
4. Watkins PB. (1990) The role of cytochrome P-450 in
cyclosporine metabolism. J. Am. Acad. Dermatol. 23:1301-1311.
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