|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. Research that led to the drug's development was financed by a $3,500,000 federal grant through the National Institutes of Health (NIH) and over $200,000,000 by Abbott Labs. Most of the $200,000,000 figure cited by Abbott paid for clinical trials—despite NIH offering to pay for them—because Abbott was concerned about "public interest" responses to the high prices they projected Norvir would command. The Food and Drug Administration (FDA) approved ritonavir on March 1, 1996, making it the seventh approved antiretroviral drug in the United States. 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 PI's 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.
Drug interactions 
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
- 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
Side effects 
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.
Appearance of a second crystal form and temporary withdrawal from the market 
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.
- James Love (2004-06-03). "How much has the public invested in ritonavir, and how much has Abbott?". Essential Inventions. Retrieved 2008-05-06..
- Ceci Connolly (2004-08-05). "NIH Declines to Enter AIDS Drug Price Battle". Washington Post. Retrieved 2006-01-16.
- Zeldin RK, Petruschke RA (2004). "Pharmacological and therapeutic properties of ritonavir-boosted protease inhibitor therapy in HIV-infected patients". Journal of Antimicrobial Chemotherapy 53 (1): 4–9. doi:10.1093/jac/dkh029. PMID 14657084.
- Merry, Concepta; Barry, Michael G.; Mulcahy, Fiona; Ryan, Mairin; Heavey, Jane; Tjia, John F.; Gibbons, Sara E.; Breckenridge, Alasdair M.; Back, David J. (1997). "Saquinavir pharmacokinetics alone and in combination with ritonavir in HIV-infected patients". AIDS 11 (4): F29–F33. doi:10.1097/00002030-199704000-00001. PMID 9084785.
- Ritonavir, Merck Manual
- Norvir, rxlist.com
- Bauer J, et al. (2004). "Ritonavir: An Extraordinary Example of Conformational Polymorphism". Pharmaceutical Research 18 (6): 859–866. doi:10.1023/A:1011052932607. PMID 11474792.
- S. L. Morisette, S. Soukasene, D. Levinson, M. J. Cima and O. Almarsson (2003). "Elucidation of crystal form diversity of the HIV protease inhibitor ritonavir by high-throughput crystallization". Proc. Natl. Acad. Sci. USA 100 (5): 2180–4. doi:10.1073/pnas.0437744100. PMC 151315. PMID 12604798.
Further reading 
- Chemburkar, Sanjay R.; Bauer, John; Deming, Kris; Spiwek, Harry; Patel, Ketan; Morris, John; Henry, Rodger; Spanton, Stephen et al. (2000). "Dealing with the Impact of Ritonavir Polymorphs on the Late Stages of Bulk Drug Process Development". Organic Process Research & Development 4 (5): 413. doi:10.1021/op000023y.
- PubPK - Ritonavir pharmacokinetics
- Norvir (manufacturer's website)
- Ritonavir (patient information)
- Ritonavir bound to proteins in the PDB
- Norvir Prescribing Information