A drug can reach serum without establishing that its proposed brain mechanism occurred. That distinction helps interpret a negative medication trial without declaring either the entire mechanism disproved or the treatment effective despite the result.
The clinical outcome remained negative
Heinzerling and colleagues’ 2020 report randomized 125 adults with methamphetamine use disorder to oral ibudilast 50 mg twice daily (64 participants) or placebo (61) for 12 weeks, with medication-management counseling.
End-of-treatment abstinence was 14% with ibudilast and 16% with placebo, without a significant difference. Classification required an available specimen in each of weeks 11 and 12 and no positive specimen in either week; missing both specimens in either week meant failure. Serum ibudilast at week three was available for 59 treated participants. Levels did not significantly correlate with the treatment-effectiveness score (r=0.008, p=1.00).
Exposure, adherence and target engagement
The authors identified limited adherence, a population with frequent baseline use and power to detect a moderate rather than every smaller effect as limitations. The trial did not establish that ibudilast improved abstinence.
A serum measurement is an observation of exposure in that specimen at that time. It cannot independently document every dose over the preceding or following weeks. Calling the level proof of continuous adherence would change the time interval covered by the evidence.
Target engagement is a further question: did the relevant biological target respond in the intended tissue? Measuring serum drug does not itself measure a change in brain inflammatory activity. A credible mechanism claim needs evidence directed at that target, alongside an appropriate account of where and when it was measured.
What does the null trial say about neuroinflammation?
The randomized clinical comparison addresses the tested treatment regimen and outcome. It does not isolate every action of a compound or establish that one proposed mechanism caused the null result. Conversely, a plausible mechanism cannot substitute for a favorable clinical comparison.
The abstinence definition also deserves to remain intact. It was a rule about available urine observations in two final weeks. It should not become a claim of uninterrupted biologically observed abstinence throughout the entire treatment period.
For an expert opinion, three questions should therefore be stated separately: whether the medicine was detected, whether its intended target was affected, and whether the clinical outcome improved. Evidence for one question should not be borrowed to answer the others. This historical outpatient trial supplies a clear negative abstinence comparison while leaving mechanistic questions for appropriately designed research.
Source
Heinzerling KG, et al. Randomized, placebo-controlled trial of targeting neuroinflammation with ibudilast to treat methamphetamine use disorder. J Neuroimmune Pharmacol. 2020;15:238–248. doi:10.1007/s11481-019-09883-w. Full text.