|Systematic (IUPAC) name|
|Pregnancy cat.||B (U.S.)|
|Legal status||℞-only (U.S.)|
|Mol. mass||720.946 g/mol|
| (what is this?)
Ritonavir is frequently prescribed with HAART, not for its antiviral action, but as it inhibits the same host enzyme that metabolizes other protease inhibitors. This inhibition leads to higher plasma concentrations of these latter drugs, allowing the clinician to lower their dose and frequency and improving their clinical efficacy.
Ritonavir is manufactured as Norvir by Abbott Laboratories. The Food and Drug Administration (FDA) approved ritonavir on March 1, 1996, making it the seventh approved antiretroviral drug and the second approved protease inhbitor in the United States. Within 2 years of the approval of ritonovir (and of saquinavir a few months earlier), the U.S. HIV-associated death rate fell from over 50,000 per year to about 18,000. 
In 2003, Abbott raised the price of a Norvir course from USD $1.71 per day to $8.57 per day, leading to claims of price gouging by patients' groups and some members of Congress. Consumer group Essential Inventions petitioned the NIH to override the Norvir patent, but the NIH announced on August 4, 2004 that it lacked the legal right to allow generic production of Norvir.
Mechanism of action
Ritonavir was originally developed as an inhibitor of HIV protease. It is one of the most complex inhibitors. It is now rarely used for its own antiviral activity, but remains widely used as a booster of other protease inhibitors. More specifically, ritonavir is used to inhibit a particular liver enzyme that normally metabolizes protease inhibitors, cytochrome P450-3A4 (CYP3A4). The drug's molecular structure inhibits CYP3A4, so a low dose can be used to enhance other protease inhibitors. This discovery, which has drastically reduced the adverse effects and improved the efficacy of protease inhibitors and HAART, was first communicated in an article published in the journal AIDS in 1997 by researchers at the University of Liverpool. This effect does come with a price: it also affects the efficacy of numerous other medications, making it difficult to know how to administer them concurrently. In addition it can cause a large number of side-effects on its own.
Concomitant therapy of ritonavir with a variety of medications may result in serious and sometimes fatal drug interactions. Ritonavir induces CYP 1A2 and inhibits the major P450 isoforms (3A4 and 2D6).
The list of clinically significant interactions of ritonavir includes but is not limited to following drugs:
- amiodarone - decreased metabolism, possible toxicity
- midazolam and triazolam - contraindicated
- carbamazepine - decreased metabolism, possible toxicity
- cisapride - decreased metabolism, possible prolongation of Q-T interval and life-threatening arrythmias
- disulfiram (with ritonavir oral preparation) - decreased metabolism of ritonavir
- flecainide - decreased metabolism, possible toxicity
- MDMA - decreased metabolism, can sometimes result in toxic outcomes like serotonin syndrome which can be life threatening
- meperidine (pethidine) - build-up of toxic concentrations of norpethidine possible
- St John's wort
- statins - decreased metabolism, without dosage modification increased risk of rhabomyolisis
- voriconazole - ritonavir increases metabolism of voriconazole
- Phenytoin - The proposed mechanism involves ritonavir induction of phenytoin metabolism via CYP450 2C9
The most common side effects of ritonavir therapy are:
- asthenia, malaise
- nausea and vomiting
- abdominal pain
- taste abonormality
One of ritonavir's side effects is hyperglycemia. It appears that ritonavir directly inhibits the GLUT4 insulin-regulated transporter, keeping glucose from entering fat and muscle cells. This can lead to insulin resistance and cause problems for Type Ⅱ diabetics.
Polymorphism and temporary market withdrawal
Norvir was originally dispensed as an ordinary capsule, which did not require refrigeration. This was as a crystal of what is now called form I. However, like many drugs, ritonavir exhibits polymorphism, i.e., the same molecule crystallizes into more than one type of crystal. The different crystals, or polymorphs, are made of the same molecules but in different crystalline arrangements. The solubility and hence the bioavailability is very different in the two different arrangements.
During development (it was introduced in 1996), only the polymorph now called form I was found, but in 1998, a lower free energy, more stable polymorph (form II) appeared. This more stable (and so less soluble) crystal form compromised the oral bioavailability of the drug. This caused the removal of the oral capsule formulation from the market.
Even a trace of form II can catalyse the transformation from the more bioavailable form I to form II. Thus form II threatened existing supplies of ritonavir as the lower solubility polymorph caused the therapeutically effective polymorph to convert to form II. Form II, which was not therapeutically effective because of poor solubility and resulting much lower bioavailability, entered production lines and effectively halted production processes.
After this discovery in the late 1990s, Abbott withdrew the original capsules from the market, and recommended patients switch to Norvir suspension while researchers worked to solve the problem. The capsules have been replaced with refrigerated gelcaps, to solve the crystallization problem of the original capsules.
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- Norvir (manufacturer's website)
- Ritonavir (patient information)
- Ritonavir bound to proteins in the PDB
- Norvir Prescribing Information