Protease inhibitors

Protease inhibitors are a structurally heterogeneous class of antiviral agents that, unlike reverse transcriptase inhibitors, act at the final stage of HIV reproduction.

Viral protease is activated at the stage of virion reproduction. Aspartate protease acts like scissors, cutting protein strips into mature viral particles, which are then released from the infected HIV-reproducing cell. Protease inhibitors bind to the active site of the enzyme, preventing the formation of full-fledged viral particles capable of infecting other cells.

This class of antiretroviral agents is currently considered the most active against HIV infection. Treatment with these drugs leads to positive dynamics of surrogate markers of infection (increase in the number of CO4+ cells and decrease in the concentration of the virus in the blood, i.e. the viral load), in addition, their use gives patients clinical advantages - reduces mortality and the frequency of clinical conditions that determine the diagnosis of AIDS. Protease inhibitors exhibit antiviral activity both in lymphocytes and in monocytic cells. Their advantage is their activity against HIV isolates resistant to zidovudine. To provide an antiviral effect, protease inhibitors, unlike nucleoside analogues, do not require intracellular metabolism, so they retain a long-term effect in chronically infected cells.

Currently, 4 HIV protease inhibitors are used in world practice: saquinavir (Invirase), indinavir (Crixivan), nelfinavir (Viracept), ritonavir (Norvir).

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Saquinavir

Saquinavir (Invirase; Hoffmann La-Roche) was the first protease inhibitor approved for use in HIV infection and is the most potent of them, inhibiting syncytium formation in vitro and improving the function of antigen-bearing dendritic cells, suggesting that the drug may restore immune status.

Saquinavir is metabolized by the cytochrome P450 enzyme system. Inducers of this system's enzymes, as well as rifampicin, inhibit activity. Saquinavir exhibits pronounced antiviral activity in combination with AZT, zalcitabine (ddC), as well as with lamivudine and stavudine. It is effective and well tolerated by both patients starting therapy and those who have already received nucleoside analogues. It has been established that the combination of saquinavir, zidovudine and zalcitabine has synergistic activity in vitro, reduces the development of resistance to each of these drugs.

A study of the efficacy of this protease inhibitor in 97 patients with triple therapy: retrovir 200 mg x3 times a day, zalcitabine 750 mg x3 times a day, saquinavir 600 mg x 3 times a day showed the most favorable dynamics of tritherapy compared to mono- and bitherapy. At the same time, an increase in the number of CD4 cells, a significant decrease in the viral load and the absence of noticeable signs of toxicity were noted. It should be taken into account that, unlike retrovir, protease inhibitors, as well as most other reverse transcriptase inhibitors, poorly penetrate the blood-brain barrier, and therefore the appointment of retrovir is mandatory.

Saquinavir in gel form (SYC), produced under the name Fortovase, has high bioavailability compared to the solid form of the drug (HGC). It is used in a dose of 1200 mg x 3 times a day or 1600 mg 2 times a day in combination with ritonavir 400 mg 2 times a day. The simultaneous use of the saquinavir / ritonavir combination (400 mg / 400 mg) provides ease of dosing - 2 times a day, recommended for first-line therapy. Special studies have shown that when using Retrovir, Epivir and Fortovase, the viral load decreases significantly faster than when using Crixivan.

In 1999, a new dosing regimen for Fortovase was established. The new treatment regimen, in which the protease inhibitor Fortovase (saquinavir) is administered once daily in combination with minimal doses of ritonavir (another protease inhibitor), allows therapeutic concentrations of saquinavir to be maintained throughout the 24-hour dosing interval. Fortovase is administered at a dose of 1600 mg per day + ritonavir 100 mg per day.

According to A.V. Kravchenko et al., 2002, combination therapy with antiretroviral drugs Fortovaza/Norvir + Nikavir + Videx in HIV-infected patients for 24 weeks was effective: a decrease in HIV RNA levels by 2.01 log/l was achieved, and in 63% of patients it was below the test system detection level (400 copies per ml), the median CD4 lymphocyte count increased by 220 cells per 1 mm%, and the immunoregulatory coefficient (CD4/8 ratio) increased significantly. The authors showed that the use of an enhanced HIV protease inhibitor (a combination of Fortovaza/Norvir) in minimal daily doses in the therapeutic regimen for 6 months has virtually no effect on lipid metabolism indices. The use of Fortovase together with one capsule of Norvir per day allows to reduce the daily dose of Fortovase to 8 capsules (instead of 18), to reduce the frequency of taking the HIV protease inhibitor to 1 time per day (instead of 3) and to reduce the monthly cost of the protease inhibitor by almost 2 times. The scheme including Fortovase/Norvir, Nikavir and Videx can be recommended as a 1st level therapy for the treatment of HIV-infected patients.

Nelfinavir

Nelfinavir (Viracept; Roche-Agouion Pharmaceuticals) is an antiretroviral drug recommended for the treatment of HIV infection in both adults and children. It is active against both HIV-1 and HIV-2.

These antiretroviral drugs are available in the following dosage forms: 250 mg tablets, 250 mg film-coated tablets, 50 mg/1 g oral powder.

The recommended doses for adults are 750 mg x 3 times a day or 1250 mg 2 times a day, for children - 20-30 mg/kg body weight x 3 times a day. The bioavailability of nelfinavir when taken orally is up to 80%.

A high therapeutic effect was achieved with a combination of nelfinavir with zidovudine, lamivudine and stavudine; combined use with other nucleoside RT inhibitors, in particular, with abacavir, protease inhibitors - saquinavir, indinavir, ritonavir, amprenamir and NNIO'G - delavirdine, nevirapine, lorivid, efavirenz is being studied.

Controlled clinical trials of nelfinavir (Viracept) in combination with other antiviral agents lasting at least 1 year have demonstrated sustained reductions in plasma HIV-1 RNA levels and increases in CD4 cell counts in both previously untreated and previously treated HIV-1-infected patients.

Nelfinavir inhibits the cytochrome P450 system, therefore, it is not recommended to take other common drugs that use the cytochrome system for metabolism, including terfenadine, cipradine, triazolam, rifampin, etc. Carbamazepine, phenobarbital, fenithione can reduce the concentration of nelfinavir in plasma, on the contrary, indinavir, saquinavir, rigonavir can increase it. When taken together with didanosine, nelfinavir should be taken two hours before or one hour after didanosine.

When nelfinavir is used alone, viral resistance develops fairly rapidly; however, when combined with nucleoside analogues, resistance may be delayed. For example, of 55 patients receiving nelfinavir alone or in combination with AZT and ZTS, resistance developed in 56% of recipients of nelfinavir alone and in 6% of recipients of combination therapy. Resistance to nelfinavir may not cause cross-resistance with other protease inhibitors.

Most of the side effects observed in clinical studies were mild. The most common side effect with nelfinavir at recommended doses was diarrhea. Other possible side effects include rash, flatulence, nausea, decreased neutrophil count, increased creatinine kinase and ALT/AST.

Nelfinavir is metabolized and eliminated primarily by the liver. Therefore, caution should be exercised when prescribing the drug to patients with impaired liver function.

Advantages of using Viracept (Nelfinavir) in first-line HAART regimens:

• mutation in codon D30N
• the main one in nelfinavir therapy,
• D30N results in decreased viral viability and does not cause cross-resistance with other PIs,
• In patients previously treated with nelfinavir, the use of other PIs in second-line regimens is effective.

Ritonavir

Ritonavir (Norvir; Abbott Laboratories) has shown the best efficacy when used at a dose of 600 mg x twice daily. These antiretroviral drugs can be used as monotherapy or in combination with nucleoside analogues. Studies by Danner et al., 1995, demonstrated a dose-dependent reduction in viral load and an increase in CD4+ cell counts with ritonavir treatment for 16-32 weeks. Cameron et al., 1996, presented the results of large clinical trials demonstrating a slowing of disease progression and a reduction in mortality in AIDS patients who were given ritonavir in addition to standard nucleoside analogue therapy. Preliminary data have shown that ritonavir can be used for initial therapy concomitantly with Norvir and zalcitabine (ddC) or lamivudine. Mellors et al., Molla et al. demonstrated high efficacy of combined use of ritonavir and saquinavir, with a significant reduction in viral load and an increase in CD4 cell count.

Ritonavir inhibits cytochrome P450 enzymes and alters plasma concentrations of many drugs, so some medications must be excluded and doses of others must be adjusted when used concomitantly with ritonavir.

Ritonavir administration may be associated with adverse events such as allergic reactions, nausea, vomiting, diarrhea, anorexia, paresthesia, asthenia, liver function tests changes, and diabetes, which are common to all approved protease inhibitors.

Resistance to ritonavir often leads to resistance to indinavir, and less commonly to nelfinavir.

Indinavir

Indinavir (crixivan; Merck) has an advantage over saquinavir and iritonavir: due to low protein binding, it reaches higher concentrations in plasma, tissues, and penetrates the central nervous system. The recommended dose is 2400 mg/day (800 mg x 3 times), indinavir is taken on an empty stomach 1 hour before or 2 hours after meals, oral bioavailability is 65%. The possibility of using the drug in children is being studied.

Indinavir significantly reduces viral load and increases CD4+ cell counts when used alone or in combination with nucleoside analogues. However, many studies confirm the greatest effect of crixivan in combination therapy.

Resistance to indinavir develops quite quickly, but to a lesser extent in those patients who began taking indinavir in combination with other antiretroviral agents and had not previously received anti-HIV therapy. Indinavir-resistant HIV-1 strains are capable of exhibiting pronounced resistance to other protease inhibitors - ritonavir, nelfinavir, and to a lesser extent - to saquinavir.

Indinavir inhibits cytochrome P450, so it is necessary to avoid its combined use with other drugs that use the cytochrome P450 system for metabolism. Didanosine reduces the absorption of indinavir, so it is recommended to take these two drugs separately at an interval of 1 hour. Ketoconazole inhibits the metabolism of indinavir, and therefore the dose of indinavir should be reduced to 600 mg x 3 times a day. In turn, indinavir inhibits the metabolism of rifabutin, which requires a 50% reduction in the dose of rifabutin.

When taking indinavir, such undesirable complications as diabetes, hemolytic anemia, as well as nephrolithiasis and dysuria, which are associated with the ability of indinavir to form crystals in the urine, may be observed.

Novel potential HIV-1 and HIV-2 protease inhibitors

Amprenavir (141W94) - antiretroviral drugs, the newest potential inhibitors of HIV-1 and HIV-2 protease, developed by GlaxoSmithKline, approved for use in the RP. It has good oral bioavailability (>70%), is characterized by a long half-life (about 7 hours), is prescribed at a dose of 1200 mg 2 times a day regardless of food intake. It is metabolized, like other protease inhibitors, by the cytochrome P450 system. It has a good therapeutic effect in triple therapy with AZT and ZTS. Combinations with other protease inhibitors (fortovase, indinavir, nelfinavir) were studied - in all cases, a significant decrease in viral load was noted (AIDS Clinical Care). Amprenavir and ritonavir regimen: Amprenavir 600 mg + ritonavir 200 mg twice daily for patients with unsuccessful 3-drug combination. Amprenavir and ritonavir were administered with two or three other antiviral drugs. Reducing the doses of amprenavir and ritonavir due to their combination reduced the toxic effect of each drug and was effective according to clinical and laboratory data (viral load decreased by 2 times compared to baseline after 2.5 months from 4.86 x 1010 log to 2.95 x 1010 log, CD4 increased from 187 to 365 x 106 log/l. Mild side effects included diarrhea, increased cholesterol and triglyceride levels.

Boehringer Ingelheim introduces a new protease inhibitor, tapranavir. Tipranavir is currently in phase II development. It is the first antiretroviral drug from a new class of non-peptide protease inhibitors. Clinical trials show that the main side effects of the drug are gastrointestinal manifestations, especially diarrhea, which is usually successfully treated.

New antiretroviral drugs have been proposed - lotshavir, which is a protease inhibitor and significantly reduces the level of viral load. Lopinavir in combination with another protease inhibitor - ritonavir is called Kaletra. Kaletra is the first combination drug from the class of HIV protease inhibitors, manufactured by Abbott Laboratories. The combination of 133.3 mg of lopinavir and 33.3 mg of ritonavir in one Kaletra capsule (80 mg of lopinavir and 20 mg of ritonavir in 1 ml of oral solution) allows achieving high, long-lasting concentrations of lopinavir in the blood plasma, which provide a powerful antiviral effect of the drug when taken at a dose of 400/100 mg 2 times a day.

When Kaletra was administered in combination with 2 NRTIs (d4T and 3TC) to antiretroviral-naive patients, after 144 weeks of treatment, a reduction in HIV RNA levels to less than 400 copies per ml of plasma was observed in 98% (RT analysis). Moreover, the increase in CD4 cell count in patients with initially low CD4 lymphocyte counts (less than 50 cells per mm1) was significant in the Kaletra group - 265 cells (nelfinavir group - 198 cells).

In patients previously treated with at least one HIV protease inhibitor (Study 765), after 144 weeks of therapy with Kaletra in combination with nevirapine and 1 NRTI, 86% and 73% of cases showed a decrease in HIV RNA to less than 400 and 40 copies per 1 ml of plasma, respectively (RT analysis).

Interaction with other drugs:

• an increase in the dose of Kaletra to 533 mg/133 mg (4 capsules or 6.5 ml) 2 times a day during meals when taken with nevirapine or efavirenz is performed in patients who have a clinically predictable decrease in the sensitivity of the virus to lopinavir (based on treatment results or laboratory data).
• The dosage of other PIs should be reduced when taken with Kaletra. Based on a limited number of observations, the dosage of amprenavir is 750 mg twice daily, indinavir 600 mg twice daily, saquinavir 800 mg twice daily when taken with Kaletra. Optimal doses of other PIs in combination with Kaletra have not been determined taking into account their safety and efficacy.
• It is recommended to reduce the daily dose of rifabutin (300 mg per day) to 75% (maximum dose 150 mg every other day or 150 mg 3 times per week). When prescribing such a combination, careful monitoring for the development of adverse events is necessary. It may be necessary to further reduce the dose of rifabutin.
• Careful monitoring is required when co-administering Kaletra with HMG-CoA reductase inhibitors: pravastatin, fluvastatin, or minimal doses of atorvastatin and cerivastatin.

In Study 863, 9% of patients treated with Kaletra experienced increases in cholesterol (>300 mg/dL) and triglycerides (>750 mg/dL).

In patients with HIV infection and hepatitis B or C, Kaletra should be prescribed with caution, since there is evidence that after 60 weeks of treatment, ALT levels increased in 12% of cases (in patients without viral hepatitis - in 3% of cases), which was completely comparable with the frequency of ALT increases in patients with HIV infection and chronic hepatitis B and C who received nelfinavir, the safest antiretroviral drugs from the class of HIV protease inhibitors - 17%.

Pancreatitis has been observed in patients taking Kaletra. In some cases, triglyceride levels have been increased. Although a causal relationship between Kaletra and pancreatitis has not been proven, increased triglyceride levels in the blood may indicate an increased risk of pancreatitis. If a patient complains of nausea, vomiting, abdominal pain, or if elevated amylase or lipase levels are detected in the serum, treatment with Kaletra and/or other antiretroviral drugs should be discontinued. Hyperglycemia, diabetes mellitus, and increased bleeding (in patients with hemophilia) have been reported in patients receiving PI treatment.

Kaletra should be used with caution in patients with liver disease, including viral hepatitis B, C, and with elevated aminotransferase levels.

Release forms:

• Soft gelatin capsules: The recommended adult dose is 3 capsules 2 times daily with meals, each Kaletra capsule contains 133.3 mg lopinavir and 33.3 mg ritopavir.
• Oral solution: The recommended dose of Kaletra solution for oral administration for adult patients is 5 ml 2 times a day during meals, the recommended dose for children from 6 months to 12 years is determined in accordance with the child's body surface area.
• Each 5 ml contains 400 mg of lopinavir and 100 mg of ritonavir. Taking Kaletra is easy: there are no dietary restrictions, no requirements for the amount of liquid consumed.

In pediatric practice, it is recommended that children be prescribed Kaletra (lopinavir and ritonavir) simultaneously with nevirapine.

A study reported at a conference in Glasgow by Julio Montaner is activating a regimen that includes two protease inhibitors: indinavir 1200 mg and ritonavir 100 mg, or indinavir 800 mg + ritonavir 200 mg; or saquinavir 1600 mg, ritonavir 100 mg + efavirenz 600 mg once daily, or capetra.

The pharmacokinetic profile has allowed the development of the first IP for once-daily administration (2 capsules of 200 mg) atazanavir. Under these conditions of administration, the concentration of atazanavir (zrivada) remains in the range of values exceeding 1C90 for long periods. Atazanavir has a favorable side-effect profile, rarely causes the formation of resistant forms, is safe and effective for more than 48 weeks, does not cause an increase in lipid and triglyceride levels (M. Fleip, Seventh European Symposium on HIV Treatment "For the Rest of Life", Budapest, February 1-3, 2002).

Thus, atazanavir:

• powerful, safe and well tolerated,
• in terms of antiviral activity it is close to nelfinavir,
• can be combined with all basic NRTI regimens,
• the smallest number of tablets taken compared to other IPs,
• Unlike other IPs, it does not cause an increase in lipid levels,
• resistance profile not identical to that of other IPs.

New candidates that could replace protease inhibitors include ABT 378 and tipranavir.

Tipranavir is a new class of non-peptide HIV-1 protease inhibitors. These protease inhibitors have demonstrated excellent activity against a variety of HIV-1 laboratory strains and patient isolates, including those resistant to the HIV nucleoside reverse transcriptase inhibitors zidovudine and delavirdine. Previous experiments have shown that the combination of tipranavir with ritonavir additionally exhibits a moderate synergistic antiviral effect against ritonavir-susceptible HIV isolates and strong synergy against ritonavir-resistant isolates.

Tipranavir maintained consistent antiviral activity against protease inhibitor-resistant HIV clinical isolates and may be useful for combination with other antiretroviral agents in regimens for patients who have failed protease inhibitor-containing therapy.

Another potent nucleoside is adefovir, to which many nucleoside-resistant strains are susceptible.

The role of immunostimulants such as interleukin 2 in immune system remodeling requires further study.

The conducted studies have shown the high efficiency of the new non-nucleoside reverse transcriptase inhibitor (NNRTI) – TMS 125. These antiretroviral drugs are derivatives of diazyl-pyrimidine. Its great advantage is the ability to affect HIV strains with key mutations to NNRTI – K103NL1001. TMS 125 has a pronounced suppression on HIV, significantly suppressing viral replication with minor side effects. A 7-day course of monotherapy was administered to previously untreated patients. Side effects of TMS 125:

• Dyspepsia - (8.3%)
• Headache - (8.3%)
• Rash - (8.3%)
• Increased ALT (125-250 units) - (8.3%)
• Bilirubinemia (22-31 µmol/l) - (8.3%)

Fusion inhibitors will be potentially active. Antiretroviral drugs T-20 (Enfuvirtide) are in clinical trials. Potential advantages of fusion inhibitors: effectiveness, safety, lack of cross-resistance. Potential disadvantages: parenteral administration, antibody formation, high cost. T-20 fuses with gp 41 - a surface marker of HIV - and thus makes it impossible for HIV to bind to cells with the CD4 receptor. It is important to note that T-20 (enfuvirtide) is synergistic in its action with nucleoside and non-nucleoside reverse transcriptase inhibitors, as well as with protease.