Pharmacokinetics
The bioavailability of fluoxetine is relatively high (72%), and peak plasma concentrations are reached in 6 to 8 hours. It is highly bound to plasma proteins, mostly albumin.
Fluoxetine is metabolized in the liver by isoenzymes of the cytochrome P450 system, including CYP2D6. The role of CYP2D6 in the metabolism of fluoxetine may be clinically important, as there is great genetic variability in the function of this enzyme among people. Only one metabolite of fluoxetine, norfluoxetine (N-demethylated fluoxetine), is biologically active.
The extremely slow elimination of fluoxetine and its active metabolite norfluoxetine from the body distinguishes it from other antidepressants. With time, fluoxetine and norfluoxetine inhibit their own metabolism, so fluoxetine elimination half-life changes from 1 to 3 days, after a single dose, to 4 to 6 days, after long-term use. Similarly, the half-life of norfluoxetine is longer (16 days) after long-term use. Therefore, the concentration of the drug and its active metabolite in the blood continues to grow through the first few weeks of treatment, and their steady concentration in the blood is achieved only after four weeks. Moreover, the brain concentration of fluoxetine and its metabolites keeps increasing through at least the first five weeks of treatment. That means that the full benefits of the current dose a patient receives are not realized for at least a month since its initiation. For example, in one 6-week study, the median time to achieving consistent response was 29 days. Likewise, complete excretion of the drug may take several weeks. During the first week after the treatment discontinuation, the brain concentration of fluoxetine decreases only by 50%, The blood level of norfluoxetine 4 weeks after the treatment discontinuation is about 80% of the level registered by the end of the first treatment week, and 7 weeks after the discontinuation norfluoxetine is still detectable in the blood.
A PET study compared the action of a single dose of fluoxetine on exclusively heterosexual and exclusively homosexual men who attested that their past and present sexual behavior, desires, and fantasies were directed entirely toward women or men, respectively. The study found that in some areas of the brain the metabolic response in these two groups was different. "Both groups, however, did exhibit similar widespread lateralized metabolic responses to fluoxetine (relative to placebo), with most areas of the brain responding in the same direction." They "did not differ on behavioral measures or blood levels of fluoxetine".
Fluoxetine is a selective serotonin reuptake inhibitor and does not appreciably inhibit norepinephrine and dopamine reuptake. Nevertheless, Ely Lilly researchers found that a single injection of a rat with a large dose of fluoxetine also results in a significant increase of brain concentrations of norepinephrine and dopamine. This effect may be mediated by 5HT2a and, in particular, 5HT2c receptors, which are inhibited by higher concentrations of fluoxetine. The Ely Lilly scientists also suggested that the effects on dopamine and norepinephrine may contribute to the antidepressant action of fluoxetine. In opinion of other researchers, however, the magnitude of this effect is unclear. The dopamine and norepinephrine increase was not observed at a smaller, more clinically relevant dose of fluoxetine. Similarly, in electrophysiological studies only larger and not smaller doses of fluoxetine changed the activity of rat's norepineprinergic neurons. Some authors, however, argue that these findings may still have clinical relevance for the treatment of severe illnesses with supratherapeutic doses (60-80 mg) of fluoxetine. Among SSRIs, 'fluoxetine is the least "selective" of all the SSRIs, with a 10-fold difference in binding affinity between its first and second neural targets (i.e., the serotonin and norepinephrine uptake pumps, respectively).' Anything greater than a 10-fold difference results in insignificant activation of the secondary neuronal targets.
Besides its well-known effects on serotonin, fluoxetine also increases density of endogenous opioid receptors in the central nervous system, which may account for some of its side effects and/or antidepressant profile.
Interactions
Fluoxetine and norfluoxetine inhibit many isozymes of the cytochrome P450 system that make drug metabolism possible. Both are potent inhibitors of CYP2D6 (the main enzyme responsible for their metabolism) and mild to moderate inhibitors of CYP1A2, CYP2B6, CYP2C9/2C19, and CYP3A4; furthermore, they inhibit the activity of P-glycoprotein, a type of membrane transport protein that plays an important role in drug transport and metabolism. This extensive effect on the body's pathways for drug metabolism creates the potential for interactions with many commonly used drugs.
The simultaneous use of fluoxetine with triptans, tramadol or other serotonergic agents can result in a rare, but potentially life-threatening adverse drug reaction called serotonin syndrome.
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