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COMPARISONS
TO OTHER DRUGS: RELATIVE POTENCY DESIGNS
Single dose relative potency
studies
Much of our information about how to use and interconvert
opioid analgesics was gathered using a class of dose-response designs called relative
potency bioassays (Laska
and Meisner, 1987). Relative potency bioassays consist of a comparison of
two or more doses of a test drug with two or more doses of a standard (Figure
6.3.1). A placebo may not be necessary in such trials, because the demonstration
of a statistically significant positive slope for the dose-response curves establishes
assay sensitivity. A placebo is necessary, however, if one wishes to estimate
the lowest dose at which analgesic efficacy might be detected.
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Figure
6.3.1
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| The
relative potency assay, a classic design well suited
to comparing side effects and analgesic potencies of
various treatments. In this study by Seed
et al., (1958)
intramuscular doses of morphine, 5
and 10 mg, were compared to dihydrocodeine, 30 and 60
mg, for pain relief (left; 5-point category scale
summed over 6 hours) and respiratory depression (right).
Relative potencies compared to morphine, denoted by
1/Æ
, were similar for
analgesia and respiratory depression. In the analgesic study
a saline placebo (horizontal dotted line) was also one of
the treatments, but was not an
essential part of the design. If
doses are chosen to ensure overlap of the analgesic
effect ranges, this design can be used to compare
side effects of a drug combination and its components
at equianalgesic doses. |
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