Penciclovir-action Mechanism

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Chain Termination Acv

FIGURE 24.12 Mechanism of antiviral action of acyclovir (ACV). ACV targets viral DNA polymerases, such as the herpesvirus (HSV) DNA polymerase. Before it can interact with viral DNA synthesis, it needs to be phosphorylated intracellularly, in three steps, to the triphosphate form. The first phosphorylation step is ensured by the HSV-encoded thymidine kinase (TK), and is therefore confined to virus-infected cells. (After De Clercq, E., Nat. Rev. Drug Discov., 1, 13, 2002.)

FIGURE 24.12 Mechanism of antiviral action of acyclovir (ACV). ACV targets viral DNA polymerases, such as the herpesvirus (HSV) DNA polymerase. Before it can interact with viral DNA synthesis, it needs to be phosphorylated intracellularly, in three steps, to the triphosphate form. The first phosphorylation step is ensured by the HSV-encoded thymidine kinase (TK), and is therefore confined to virus-infected cells. (After De Clercq, E., Nat. Rev. Drug Discov., 1, 13, 2002.)

Activity spectrum: HSV-1, HSV-2, and VZV.

Mechanism of action: Serves as oral prodrug of penciclovir (to which it is converted by hydrolysis of the two acetyl groups and oxidation at the 6-position), then acts as described for penciclovir. Principal indication(s): HSV-1, HSV-2, and VZV infections.

Administered: Orally at 750 mg/day (250 mg tablet every 8 h, three times a day), or 1500 mg/day (500 mg every 8 h).

24.4.1.5 Idoxuridine

Structure (Figure 24.11): 5-Iodo-2'-deoxyuridine (IDU, IUdR), Herpid®, Stoxil®, Idoxene®, Virudox®, etc.

Activity spectrum: HSV-1, HSV-2, and VZV.

Mechanism of action: Incorporated into (viral/cellular) DNA, following intracellular phosphoryla-tion to IDU 5'-triphosphate (in virus-infected and uninfected cells). Principal indication(s): HSV keratitis.

Administered: Topically as eyedrops (0.1%) or ophthalmic cream.

24.4.1.6 Trifluridine

Structure (Figure 24.11): 5-Trifluoromethyl-2'-deoxyuridine, trifluorothymidine (TFT), Viroptic®.

Activity spectrum: HSV-1, HSV-2, and VZV.

Mechanism of action: Inhibits conversion of dUMP to dTMP by thymidylate synthase, following intracellular phosphorylation to TFT 5'-monophosphate.

Principal indication(s): HSV keratitis.

Administered: Topically as eyedrops (1%) or ophthalmic cream. 24.4.1.7 Brivudin

Structure (Figure 24.11): (£)-5-(2-Bromovinyl)-2'-deoxyuridine, bromovinyldeoxyuridine (BVDU), Zostex®, Brivirac®, Zerpex®.

Activity spectrum: HSV (type 1), VZV, and some other (veterinarily important) herpesviruses. Mechanism of action: Targeted at the viral DNA polymerase, can act as competitive inhibitor (with respect to the normal substrate, dTTP) after intracellular phosphorylation to BVDU 5'-triphosphate; can also act as alternate substrate and be incorporated into the viral DNA, thus leading to a reduced integrity and functioning of the viral DNA (Figure 24.13). The first and second phosphorylation steps are catalyzed by the virus-encoded TK (HSV-1 TK, VZV TK), which explains the remarkable specificity of BVDU for these viruses.

Principal indication(s): HSV-1 and VZV infections, particularly herpes zoster, but also HSV-1 keratitis and herpes labialis. Brivudin has been licensed for the treatment of herpes zoster in immu-nocompetent patients in a number of European countries.

Administered: Orally at 125 mg/day, once-daily (herpes zoster); can also be administered topically, as 0.1%-0.5% eyedrops (herpetic keratitis) or 5% cream (herpes labialis).

Fusidic Acid Cream Mechanism Action

DNA template

FIGURE 24.13 Mechanism of antiviral action of BVDU. Following uptake by the (virus-infected) cells, BVDU is phosphorylated by the virus-encoded thymidine kinase (TK) to the 5'-monophosphate (BVDU-MP) and 5'-diphosphate (BVDU-DP), and further onto the 5'-triphosphate (BVDU-TP) by cellular kinases, i.e., nucleoside 5'-diphosphate (NDP) kinase. BVDU-TP can act as a competitive inhibitor/alternative substrate of the viral DNA polymerase, and as a substrate it can be incorporated internally (via internucleotide linkages) into the (growing) DNA chain. (After De Clercq, E., Biochem. Pharmacol., 68, 2301, 2004a.)

DNA template

FIGURE 24.13 Mechanism of antiviral action of BVDU. Following uptake by the (virus-infected) cells, BVDU is phosphorylated by the virus-encoded thymidine kinase (TK) to the 5'-monophosphate (BVDU-MP) and 5'-diphosphate (BVDU-DP), and further onto the 5'-triphosphate (BVDU-TP) by cellular kinases, i.e., nucleoside 5'-diphosphate (NDP) kinase. BVDU-TP can act as a competitive inhibitor/alternative substrate of the viral DNA polymerase, and as a substrate it can be incorporated internally (via internucleotide linkages) into the (growing) DNA chain. (After De Clercq, E., Biochem. Pharmacol., 68, 2301, 2004a.)

24.4.2 CMV Inhibitors 24.4.2.1 Ganciclovir

Structure (Figure 24.14): 9-(1,3-Dihydroxy-2-propoxymethyl)guanine (DHPG), (GCV), Cymevene®, Cytovene®.

Activity spectrum: HSV (types 1 and 2), CMV, and some other herpesviruses. Mechanism of action: Targeted at the viral DNA polymerase, where it mainly acts as a chain terminator, following intracellular phosphorylation to GCV triphosphate and incorporation of GCV monophosphate at the 3'-end of the viral DNA chain. First phosphorylation step is catalyzed by the HSV-encoded thymidine kinase (TK) or CMV-encoded protein kinase (PK), which explains the specificity of ganciclovir for HSV and CMV, respectively.

Principal indication(s): CMV infections, particularly CMV retinitis in immunocompromised (i.e., AIDS) patients (treatment and prevention).

Administered: Intravenously at 10 mg/kg/day (2 x 5 mg/kg, every 12 h) for induction therapy; orally at 3000 mg/day (three times four 250 mg capsules) for maintenance therapy and for prevention; intraocular (intravitreal) implant (Vitrasert*) of 4.5 mg ganciclovir as localized therapy of CMV retinitis.

OH Ganciclovir

OH Ganciclovir

H3C7 SCH,

Valganciclovir

H3C7 SCH,

Foscarnet

Cidofovir OH

5'-d-[G*C*G*T*T*T*G*C*T*C*T*T*C*T*T*C*T*T*G*C*G]-3' Sodium salt

* = Racemic phosphorothioate

Fomivirsen

FIGURE 24.14 Structures of a number of cytomegalovirus (CMV) inhibitors.

24.4.2.2 Valganciclovir

Structure (Figure 24.14): L-Valine ester of ganciclovir (VGCV), Valcyte®. Activity spectrum: As for GCV.

Mechanism of action: Serves as oral prodrug of GCV, and then acts as described for GCV. Principal indication(s): CMV infections. Oral valganciclovir is expected to replace intravenous ganciclovir in both the therapy and prevention of CMV infections.

Administered: Orally at 900 mg/day (two 450 mg tablets daily) for maintenance therapy (900 mg twice daily for induction therapy).

24.4.2.3 Foscarnet

Structure (Figure 24.14): Trisodium phosphonoformate, foscarnet sodium, Foscavir®. Activity spectrum: Herpesviruses (HSV-1, HSV-2, VZV, CMV, etc.) and also HIV.

Mechanism of action: Pyrophosphate analog, interferes with the binding of the pyrophosphate (diphosphate) to its binding site of the viral DNA polymerase, during the process of DNA polymerization. Principal indication(s): CMV retinitis in AIDS patients, and mucocutaneous acyclovir-resistant (viral TK-deficient) HSV and VZV infections in immunocompromised patients.

Administered: Intravenously at 180 mg/kg/day (3 x 60 mg/kg, every 8 h) for induction therapy of CMV retinitis; intravenously at 120 mg/kg/day (3 x 40 mg/kg, every 8 h) for maintenance therapy of CMV retinitis and for therapy of acyclovir-resistant mucocutaneous HSV or VZV infections in immunocompromised patients. Dose adjustments for changes in renal function are imperative.

24.4.2.4 cidofovir

Structure (Figure 24.14): (S)-1-(3-Hydroxy-2-phosphonylmethoxypropyl)cytosine (HPMPC), (CDV), Vistide®.

Activity spectrum: Herpesviruses (HSV-1, HSV-2, VZV, CMV, etc.), papilloma-, polyoma-, adeno-, and poxviruses.

Mechanism of action: Targeted at the viral DNA polymerase, acts as a chain terminator, following intracellular phosphorylation to the diphosphate form, and incorporation at the 3'-end of the viral DNA chain (two sequential incorporations needed for chain termination in the case of CMV DNA synthesis) (Figure 24.15).

Principal indications(s): Officially licensed for the treatment of CMV retinitis in AIDS patients. Also shown to be effective in the treatment of acyclovir-resistant (viral TK-deficient) HSV infections, recurrent genital herpes, genital warts, CIN-III (cervical intraepithelial neoplasia grade III), laryn-geal and cutaneous papillomatous lesions, molluscum contagiosum lesions, orf lesions, adenovirus infections, and progressive multifocal leukoencephalopathy (PML).

Administered: Intravenously (Vistide) at 5 mg/kg/week during the first 2 weeks, then 5 mg/kg every other week, with sufficient hydration and under cover of probenecid to prevent nephrotoxicity. It can also be administered topically as a 1% gel or cream.

24.4.2.5 Fomivirsen

Structure (Figure 24.14): Antisense oligodeoxynucleotide composed of 21 phosphorothioate-linked nucleosides, ISIS 2922, Vitravene®.

Activity spectrum: CMV.

Mechanism of action: Being complementary in base sequence, it hybridizes with, and thus blocks expression (translation) of, the CMV immediate and early 2 (IE2) mRNA.

Principal indication(s): CMV retinitis (in AIDS patients).

Administered: Intraocularly (intravitreally).

NH2 6

HO HPMPC

HO HPMPCp

HO HPMPCpp

■ Viral DNA polymerase

DNA product

DNA template

FIGURE 24.15 Mechanism of antiviral action of cidofovir (HPMPC). HPMPC needs to be phosphorylated, in two steps, to the diphosphate form before it interferes, as chain terminator (following two consecutive incorporations in the case of CMV) with the DNA polymerase reaction. (After De Clercq, E., Expert Rev. Anti-infect. Ther, 1, 21, 2003a.)

24.5 ANTI-INFLUENZAVIRUS COMPOUNDS (INCLUDING RIBAVIRIN) 24.5.1 Amantadine

Structure (Figure 24.16): Tricyclo[3.3.1.1.3,7]decane-1-amine hydrochloride, 1-adamantanamine, amantadine HCl, Symmetrel®, Mantadix®, Amantan®, etc. Activity spectrum: Influenza A virus.

Mechanism of action: Blocks M2 ion channel, and thus prevents the passage of H+ ions that are required for the necessary acidity to allow for the decapsidation (viral uncoating process).

NH2.HCl

Amantadine

NH2.HCl

Rimantadine

NH2.HCl

Rimantadine

Amantadine

CH3 NH nh2

NH Zanamivir

HO OH Ribavirin

FIGURE 24.16 Structures of a number of anti-influenza-virus compounds.

Principal indication(s): Influenza A virus infections (prevention and early therapy). Also used in the treatment of Parkinson's disease.

Administered: Orally at 200 mg/day (two times a 100 mg capsule). 24.5.2 Rimantadine

Structure (Figure 24.16): a-Methyltricyclo[3.3.1.1.3,7]decane-1-methanamine hydrochloride, a-methyl-1-adamantanemethylamine HCl, Flumadine®.

Activity spectrum: Influenza A virus.

Mechanism of action: As for amantadine.

Principal indication(s): Influenza A virus infections (prevention and early therapy). Administered: Orally at 300 mg/day (two times 150 mg).

24.5.3 Zanamivir

Structure (Figure 24.16): 4-Guanidino-2,4-dideoxy-2,3-didehydro-N-acetylneuraminic acid, 5-acetylamino-4-[(aminoiminomethyl)amino]-2,6-anhydro-3,4,5-trideoxy-D-glycero-D-galacto-non-2-enonic acid, CG 167, Relenza®.

NH2.HCl

Oseltamivir Mechanism
FIGURE 24.17 Locations of oseltamivir-resistance mutations (H274Y) showing that the tyrosine at position 252 is involved in a network of hydrogen bonds in group-1 (H5N1 and H1N1) neuraminidases. (After Russell, R.J. et al., Nature, 443, 45, 2006.)

Activity spectrum: Influenza (A and B) virus.

Mechanism of action: Zanamivir and oseltamivir (see below) are N-acetylneuraminic (sialic acid) analogues, which are targeted at the influenza viral neuraminidase (sialidase) (Figure 24.17). The viral neuraminidase is responsible for the cleavage of N-acetylneuraminic acid present in the influenza virus receptor so that progeny virus particles can be released from the infected cells. The neuraminidase inhibitors zanamivir and oseltamivir keep the virus trapped onto the surface of the cells so that they cannot be released and hence cannot infect other cells.

Principal indication(s): Influenza A and B viral infections (therapy and prevention). Administered: By (oral) inhalation, at a dosage of 20 mg/day (two times 5 mg, every 12 h) for 5 days. Treatment to be started as early as possible, and certainly within 48 h, after onset of the symptoms.

24.5.4 OSELTAMIVIR

Structure (Figure 24.16): Ethyl ester of (3R,4R,5S)-4-acetamido-5-amino-3-(1-ethylpropoxy)-1-

cyclohexane-1-carboxylic acid, GS 4104, Ro 64-0796, Tamiflu®.

Activity spectrum: Influenza (A and B) virus.

Mechanism of action: As for zanamivir.

Principal indication(s): As for zanamivir.

Administered: Orally at 150 mg/day (two times a 75 mg capsule, every 12 h) for 5 days. Treatment should be started as early as possible, and certainly within 48 h, after onset of the symptoms.

24.5.5 Ribavirin

Structure (Figure 24.16): 1-P-D-Ribofuranosyl-1H-1,2,4-triazole-3-carboxamide, Virazole®, Virazid®, Viramid®.

Activity spectrum: Various DNA and RNA viruses, in particular orthomyxoviruses (influenza A and B), paramyxoviruses (measles, RSV) and arenaviruses (Lassa, Junin, etc.). Mechanism of action: The principal target for ribavirin (in its 5'-monophosphate form) is IMP dehydrogenase, that converts IMP to XMP, a key step in the de novo biosynthesis of GTP and dGTP. In its 5'-triphosphate form, ribavirin can also interfere with the viral RNA polymerase and in the formation of 5'-capped oligonucleotide primer that is required for transcription of the influenza RNA genome.

Principal indication(s): As a small size droplet aerosol, in the treatment of RSV infections in high-risk infants, and in combination with interferon-a (Intron A®, as in Rebetron®) or pegylated interferon-a (PEG-INTRON® or Pegasys®) in the treatment of HCV infections. Administered: Orally at doses of 800-1200 mg/day, in the treatment of HCV infections; or by aerosol (solution of 20 mg/mL), which has proved superior to placebo aerosol in the treatment of RSV infections.

24.6 CURRENT STATE OF THE ART

Almost 50 antiviral compounds (not including interferons or immunoglobulins) have momentarily been licensed for the treatment of HIV, HBV, herpesvirus, influenza virus, and/or HCV infections. In the preceding sections these compounds have been discussed from the following viewpoints: chemical structure, activity spectrum, mechanism of action, principal clinical indication(s), route(s) of administration and dosage. Other points that need to be considered before the full clinical potential of any given drug could be appreciated are: (1) duration of treatment, (2) single- versus multiple-drug therapy, (3) pharmacokinetics, (4) drug interactions, (5) toxic side effects, and (6) development of resistance.

As to the duration of treatment, this may vary from a few days (HSV, VZV, influenza virus infections) to several months or years (HIV, HBV, and HCV infections), depending on whether we are dealing with an acute (primary [i.e., influenza] or recurrent [i.e., HSV, VZV]) infection or chronic, persistent (i.e., HIV, HBV, and HCV) infection.

While the short-term treatment (5-7 days) of HSV, VZV, and influenza virus infections, and even the more prolonged treatment of CMV infections, can be based on single-drug therapy, for the long-term treatment of HIV infections a combination of several drugs in a triple-drug cocktail (also referred to as HAART for "highly active antiretroviral therapy") has become the standard procedure. For HIV infection, this triple-drug combination therapy can now be given as a single, once-daily oral pill, i.e., Atripla, containing three active ingredients: TDF, emtricitabine, and efa-virenz (De Clercq, 2006b).

Pharmacokinetic parameters to be addressed, when evaluating the therapeutic potential, include bioavailability (upon either topical, oral, or parenteral administration), plasma protein binding affinity, distribution through the organism (penetration into the CNS, when this is needed), metabolism through the liver (i.e., cytochrome P-450 drug-metabolizing enzymes) and elimination through the kidney. Particularly when concocting the multiple-drug combinations for the treatment of HIV infection, possible drug-drug interactions should be taken into account: i.e., some compounds act as P-450 inhibitors and others as P-450 inducers, and this may greatly influence the plasma drug levels achieved, especially in the case of NNRTIs and PIs.

Toxic side effects, both short- and long-term, must be considered when the drugs have to be administered for a prolonged period, as in the treatment of HIV infections. These side effects may seriously compromise compliance (adherence to drug intake), and could, at least in part, be circumvented by reducing the pill burden to, ideally, once-daily dosing.

Finally, resistance development may be an important issue, again for those compounds that have to be taken for a prolonged period, as is generally the case for most of the NRTIs, NNRTIs, and PIs currently used in the treatment of HIV infections. Yet, the nucleoside phosphonate analogues (NtRTIs) tenofovir and adefovir do not readily or rapidly lead to resistance development, even after several years of therapy (as for HIV and HBV infections). Increasing resistance has been noted with HBV against lamivudine after 1 or more years of treatment, but, even if resistant to lamivu-dine, HBV infections remain amenable to treatment with adefovir dipivoxil. As has been occasionally observed in immunosuppressed patients, HSV may develop resistance to acyclovir, and CMV to ganciclovir, but, if based on ACV TK or CMV PK deficiency, these resistant viruses remain amenable to treatment with foscarnet and/or cidofovir. In immunocompetent patients, treated for an acute or episodic HSV, VZV, or influenza virus infection, short-term therapy is unlikely to engender any drug resistance problems.

24.7 ANTIVIRAL AGENTS IN (PRE)CLINICAL DEVELOPMENT

In addition to the 42 antiviral compounds that by the end of 2006 were available, there are a few more that are under clinical development, and many more that are under preclinical development.

For HIV (De Clercq, 2004), these include the virus adsorption inhibitors (cosalane derivatives, cyanovirin-N, cyclotriazadisulfonamide [CADA] derivatives, teicoplanin aglycons); the CXCR4 antagonist AMD070; the CCR5 antagonists maraviroc (UK-427857) and vicriviroc (SCH-D); the NRTIs (±)-2'-deoxy-3'-oxa-4'-thiacytidine (dOTC) (apricitabine); racemic (±)FTC, amdoxovir (diaminopurine dioxolane, DAPD), Reverset (0-D-d4FC), and alovudine (FddThd); the NNRTIs etravirine (TMC125), dapivirine (TMC120), and rilpivirine (TMC278); and the integrase inhibitors Raltegravir (MK-0518) and Elvitegravir (GS-9137). Maraviroc, etravirine, and raltegravir have, in the mean time, been licensed for clinical use in the treatment of HIV infections.

For HBV, four compounds, namely, lamivudine, adefovir dipivoxil, entecavir, and telbivudine have been licensed for medical use and TDF will most likely be the next one and has, in the mean time, been licensed for the treatment of chronic hepatitis B, and for HCV, a variety of compounds targeted at either the viral protease or RNA-dependent RNA polymerase (RdRp) are currently under intensive scrutiny. Also for the herpesviruses (i.e., HSV), new antivirals have been described that target the helicase/primase complex, terminase complex or UL97 protein kinase, and, likewise, new inhibitors are on the horizon for CMV (De Clercq, 2004b) and VZV (De Clercq, 2003c).

24.8 APPRAISAL OF CLINICAL UTILITY

Currently licensed antiviral drugs are particularly focussed on the treatment of HIV, HBV, herpes-virus, influenza virus, and HCV infections, and, so are most of forthcoming antiviral compounds that are in (pre)clinical development.

For the treatment of HIV/AIDS there are now 22 anti(retro)viral drugs available, and to achieve the largest possible benefit, these drugs have to be combined in multiple-drug regimens. Numerous drug combinations could be envisaged. Those that have been generally used consist of two NRTIs, or one NRTI, and one NtRTI (TDF), to which is then added one NNRTI or one PI. Because of the long-term side effects (such as lipodystrophy, diabetes, and cardiovascular disturbances) associated with the PIs that have been longest in use, there is a tendency for starting anti-HIV therapy with PI-sparing regimens.

One such regimen that has proven to be quite efficacious in the treatment of HIV infections, and seems to be well tolerated, is the combination of TDF with (-)FTC (emtricitabine) and efavirenz. Enfuvirtide represents a new dimension in anti-HIV therapy, which could be added onto any (optimized background) regimen, but, because of the costs involved and the fact it has to be administered subcutaneously (twice daily), enfuvirtide should be primarily reserved for salvage therapy.

For the treatment of HBV infections, four compounds, lamivudine, adefovir dipivoxil, ente-cavir, and telbivudine, besides human interferon, are currently available. TDF is recommended for use in HIV-infected patients who are coinfected with HBV and should soon become available for the treatment of chronic hepatitis B as well. Whether the treatment of HBV infections should be based upon multiple-drug regimens (so as to minimize the emergence of virus-drug resistance), as in the case of HIV/AIDS, needs to be addressed in future studies. A dual-drug regimen that could be envisaged for the treatment of chronic hepatitis B is that of TDF combined with emtricitabine.

The treatment for HSV and VZV infections is since many years fairly well consolidated: it is based on the use of acyclovir, valaciclovir, or famciclovir, and in several European countries, also brivudin (BVDU). Here, there is no need for drug combination therapy, as virus-drug resistance has only rarely proved to be a problem, and, if so (in severely immunocompromised patients), therapy could be switched to, for example, foscarnet or cidofovir. The latter two drugs, which must be administered intravenously, are also used in the treatment of CMV infections in immunosuppressed patients, mostly as a second choice following the use of (val)ganciclovir.

For the therapy and prophylaxis of influenza virus infections the neuraminidase inhibitors zanamivir and oseltamivir have acquired increased momentum (the latter being more practical as it can be administered orally whereas the former has to be administered through (oral) inhalation). These neuraminidase inhibitors may be expected to be effective against new influenza virus types or variants for which no vaccines are available. Finally, for the treatment of HCV infections, the combination of ribavirin with pegylated IFN still stands out as the current choice of treatment, although it provides a durable response only in a portion of the patients (depending on the HCV genotype).

FURTHER READINGS

De Clercq, E. 2002. Strategies in the design of antiviral drugs. Nat. Rev. Drug Discov. 1: 13-25.

De Clercq, E. 2003a. Potential of acyclic nucleoside phosphonates in the treatment of DNA virus and retrovirus infections. Expert Rev. Anti-infect. Ther. 1: 21-43. De Clercq, E. 2003b. The bicyclam AMD3100 story. Nat. Rev. Drug Discov. 2: 581-587. De Clercq, E. 2003c. Highly potent and selective inhibition of varicella-zoster virus replication by bicyclic fluro

[2,3-d]pyrimidine nucleoside analogues. Med. Res. Rev. 23: 253-274. De Clercq, E. 2004a. Discovery and development of BVDU (brivudin) as a therapeutic for the treatment of herpes zoster. Biochem. Pharmacol. 68: 2301-2315. De Clercq, E. 2004b. Antivirals and antiviral strategies. Nat. Rev. Microbiol. 2: 704-720. De Clercq, E. 2006a. Antiviral agents active against influenza A viruses. Nat. Rev. Drug Discov. 5: 1015-1025. De Clercq, E. 2006b. From adefovir to Atripla™ via tenofovir, Viread™ and Truvada™. Future Virol. 1: 709-715. De Clercq, E. and Holy, A. 2005. Acyclic nucleoside phosphonates: A key class of antiviral drugs. Nat. Rev. Drug Discov. 4: 928-940.

*De Clercq, E. 2009. Anti-HIV drugs: 25 compounds approved within 25 years after the discovery of HIV. Int.

I. Antimicrob. Agents, 33, 307-320. Doms, R.W. 2000. Beyond receptor expression: The influence of receptor conformation, density, and affinity in

HIV-1 infection. Virology 276, 229-237. Gallant, J.E., DeJesus, E., Arribas, J.R. et al. 2006. Tenofovir DF, emtricitabine, and efavirenz vs. zidovudine, lamivudine, and efavirenz for HIV. N. Engl. J. Med. 354, 251-260. Hadziyannis, S.J., Tassopoulos, N.C., Heathcote, E.J. et al. 2005. Long-term therapy with adefovir dipivoxil for

HBeAg-negative chronic hepatitis B. N. Engl. J. Med. 352: 2673-2681. Pauwels, R. 2004. New non-nucleoside reverse transcriptase inhibitors (NNRTIs) in development for the treatment of HIV infections. Curr. Opin. Pharmacol. 4: 437-446. Pauwels, R. 2006. Aspects of successful drug discovery and development. Antiviral Res. 71, 77-89. Russell, R.J., Haire, L.F., Stevens, D.J. et al. 2006. The structure of H5N1 avian influenza neuraminidase suggests new opportunities for drug design. Nature 443, 45-49.

* At present, 25 compounds have been approved for the treatment of HIV infections.

25 Antibiotics

Piet Herdewijn

CONTENTS

25.1 Introduction 420

25.2 Antibiotics Affecting Bacterial Cell Wall Formation 420

25.2.1 ß-Lactam Antibiotics 420

25.2.1.1 General 420

25.2.1.2 Penicillins 421

25.2.1.3 Cephalosporins 423

25.2.1.4 Nontraditional ß -Lactam Antibiotics 427

25.2.2 Bacitracin 429

25.2.3 Vancomycin and Related Products 430

25.3 Antibiotics Affecting the Cytoplasmic Membrane 430

25.3.1 Antibiotics that Disorganize Membrane Structure 430

25.3.1.1 Tyrothricin 430

25.3.1.2 Polymyxin 431

25.3.2 Antibiotics with Altered Membrane Permeability 431

25.3.2.1 Gramicidins 431

25.3.2.2 Polyether Antibiotics 431

25.4 Antibiotics Affecting Nucleic Acid Synthesis 433

25.4.1 Actinomycin and Anthracyclines 433

25.4.2 Antibiotics that Inhibit RNA Polymerase 433

25.4.2.1 Novobiocin 434

25.4.2.2 Quinolones 434

25.5 Antibiotics Affecting the Protein Synthesis 434

25.5.1 Inhibitors of the 30 S Ribosomal Subunit 434

25.5.1.1 Aminoglycosides 434

25.5.1.2 Tetracyclines 439

25.5.2 Inhibitors of the 50 S Ribosomal Subunit 440

25.5.2.1 Chloramphenicol 440

25.5.2.2 Macrolides 440

25.5.2.3 Lincomycin, Clindamycin 441

25.5.2.4 Fusidic Acid 441

25.6 Cell Walls of Bacteria 441

25.7 Membranes 444

25.8 Nucleic Acid Synthesis 444

25.9 Protein Synthesis 445

25.10 New Developments in Antibacterial Research 445

25.11 Concluding Remarks 447

Further Readings 447

25.1 INTRODUCTION

The story of the present-day antibiotics is mainly an old story, as few new entities reached the market during the last decennium. However, the increasing clinical problems encountered with the appearance of resistant strains of bacteria have renewed interest in the field. It is therefore time for a historical overview and for some future perspectives.

Vuillemin first used the word "antibiotic" in 1889 to describe antagonism between living organisms. Waksman formulated the present definition in 1942 as "a chemical substance produced by microorganisms that can inhibit the growth or even destroy other microorganisms." Antibacterial substances found, e.g., in plants were not ranked antibiotics.

The discovery, by Fleming in 1929, of an antibacterial substance produced by a Penicillium is well known. It should be mentioned that the production of antibacterial substances by various microorganisms has been described by several authors, e.g., products from various bacteria active against Anthrax by Pasteur and Joubert in 1877; pyocyanase from Pseudomonas aeruginosa (previous name: Bac. pyocyaneus) by Emmerich and Low in 1899; cultures of Actinomyces by Gratia and Dath 1925; and "tyrothricin" by Dubos in 1939. All these products were either too toxic for systemic application or presented a low activity. The treatment of infections by these substances was doubtful and even Fleming wrote in 1941 in a letter to the Brit. Med. J. that he had used cultures of Penicillium in a few cases of septic wounds, but that, although the results were reasonably good, the trouble of making the product seemed not worthwhile. The discovery of the antibiotic penicillin has been made by H. Florey and his collaborators, Chain, Abraham, and Heatley, who produced the product by fermentation, isolated it, and showed its activity and innocuity in animals and humans. The great effort in England and the United States to produce sufficient quantities of penicillin in the years 1942-1945 is well known. The structure of penicillin was elucidated by a great number of investigators in these two countries.

The discovery of penicillin led the soil microbiologist Selman A. Waksman at the New Jersey Agricultural Experiment Station of Rutgers University, New Brunswick, NJ, to examine his large collection of Streptomyces for the production of antibiotics. In 1944, his group announced the discovery of streptomycin. The genus Streptomyces has been the most important source of therapeuti-cally useful antibiotics.

Important antibiotics are produced by fungi, which are plant-like, nonphotosynthetic eukary-otes, which grow in filamentous multicellular aggregates, e.g., Penicillium chrysogenum (penicillin), Cephalosporium (cephalosporins), and Penicillium griseofulveum (griseofulvin), which belong to the class of the Fungi imperfecti (Deuteromycetes).

Other important genera are Streptomyces (streptomycin, neomycin, chloramphenicol, tetracy-clines, and macrolides), Nocardia (rifamycins and nocardicin), and Micromonospora (gentamicin, sisomicin). They belong to the family of the Actinomycetales, which are Gram-positive bacteria that grow in the form of mycelia. Some antibiotics like polymyxin, tyrothicin, and bacitracin are produced by the bacteria of the genus Bacillus, which are also prokaryotes. Subsequently, b-lactam antibiotics have been discovered in Streptomyces, e.g., cephamycins (7-methoxycephalosporins), thienamycin, and other carbapenems, clavulanic acid, monobactams in Chromobacterium viola-ceum, Gluconobacter, Acetobacter, etc.

25.2 ANTIBIOTICS AFFECTING BACTERIAL CELL WALL FORMATION 25.2.1 b-Lactam Antibiotics

25.2.1.1 General

The structure of penicillin was determined based on a great number of studies performed in England and the United States in the years 1942-1945. After some hesitation concerning the b-lactam-thiazolidine structure, this formula was accepted based on x-ray diffraction studies of the potassium and rubidium salts of penicillin. In the cephalosporins, whose structure was determined in 1961, the thiazolidine cycle is replaced by a hydrothiazine ring. It should be noted that not only the double ring structure but also the configuration at C3, C5, and C6, respectively C6 and C7, are essential for activity.

25.2.1.2 Penicillins

The production was and still is performed by fermentation. The original Penicillium notatum of Fleming was replaced by Penicillium chrysogenum, whose production has been increased more than a 1000-fold by the isolation of mutants. Instead of surface culture, already in 1943 deep fermentation was developed in the United States. Large steel cylinders of 100.000 L and more, which are stirred and aerated are used. Culture media usually contain mineral salts, a carbohydrate source and soy or peanut meal, and/or cornsteep liquor. It was found in 1944 in a Government laboratory in Peoria, that addition of cornsteep liquor increased penicillin production. The use of cornsteep liquor was the origin of the difference between the American penicillin (penicillin G) that has a benzyl side chain and the English penicillin (penicillin F) that has a pentenyl side chain. The difference is due to the presence of phenylethylamine (formed from phenylalanine), which is transformed into phenylacetic acid. Since then, this acid is always added during penicillin G fermentation. On this basis, Behrens et al. in the Lilly laboratories, examined in 1944-1945 several acids as precursors for penicillin production. They obtained 11 new penicillins. All these penicillins were active, but apparently had no interesting new properties. In 1954, Brandl and Margreiter in Austria observed that a new penicillin, which was acid stable, was formed when phenoxyethanol was added to the culture medium. This penicillin that was identified as phenoxymethyl penicillin (penicillin V), which unexpectedly was acid stable, was introduced as oral penicillin. It is produced by adding phenoxyacetic acid to the fermentation medium of Penicillium chrysogenum.

The production of new penicillins by this method, however, is very limited because the mold incorporates only monosubstituted acetic acids (R-CH2-COOH). The possibility of preparing other penicillins was vastly increased by the discovery of 6-aminopenicillanic acid (6-APA) by Batchelor, Doyle, and Rolinson in 1959. They could isolate this substance from the fermentation media of Penicillium chrysogenum to which no precursor acid was added. The yield was very low but fortunately another method of preparation was found. The Beecham group and other laboratories in the United States and Germany (in 1960) discovered that the side chain of penicillin G could be removed by an enzyme from some bacteria. Many bacteria produce this enzyme, but only certain strains of Escherichia coli are used in the industry. It was also found that this enzyme, called penicillin acylase, did not cleave penicillin V. But an enzyme present in other microorganisms like Fusarium or Erwinia was able to transform penicillin V into 6-APA. Still another practical procedure is the chemical cleavage of penicillin to 6-APA, discovered in 1970 by Weissenburger and Vanderhoeven.

The first semisynthetic penicillins, phenethicillin, and propicillin, obtained by reaction of 6-APA with a-phenoxypropionic or a-phenoxybutyric acid, had a spectrum of activity very similar to that of phenoxymethylpenicillin (Table 25.1). A more important advance was the discovery of ampi-cillin = a-aminobenzylpenicillin by Doyle et al. in 1962 (Table 25.2). This penicillin had a broader spectrum of activity than the previous products, i.e., it is active against some Gram-negative bacteria like E. coli, Proteus, Enterococcus but not against Klebsiella, Enterobacter, Pseudomonas aeruginosa. The antibacterial spectrum of cyclacillin is similar to that of ampicillin but its activity is lower.

The p-hydroxy derivative of ampicillin, amoxicillin, unexpectedly presents a much better oral absorption than ampicillin (1971). Similarly, a better oral absorption occurs also with esters of ampicillin, pivampicillin, talampicillin, and bacampicillin (1970-1975). These prodrugs are hydro-lyzed to ampicillin, partially through the action of esterases, partially spontaneously. It also should be noticed that in these penicillins, phenyl glycine or derivative, has the D-configuration because this epimer is more active than the epimer with an L-side chain.

TABLE 25.1 Penicillins o

OCH3

OCH3

Benzylpenicillin (Penicillin G) Penicillin F

Phenoxymethylpenicillin Methicillin

Cloxacillin

Dicloxacillin

Flucloxacillin

Another important development was the discovery of penicillins resistant to penicillinase. Around 1960, quite a number of staphylococci were resistant to penicillin, because they produced an enzyme, penicillinase, which opens the ß-lactam ring. Methicillin was the first penicillin found to be resistant against Staphylococcal penicillinase. More important are the oxacillins discovered by Doyle and Nayler in the Beecham Laboratories (1952-1962), which may also be administrated orally. The introduction of one chlorine (cloxacillin) and two chlorine atoms (dicloxacillin) improves not only the oral absorption but also increases their protein binding. Flucloxacillin seems to be the best compromise for optimum activity and protein binding (Table 25.1).

Carbenicillin (1967), which contains a carboxyl group in the a-position of the side chain, was the first penicillin with antipseudomonal activity. Its activity is rather low (MIC ± 30 |g/mL) and large doses must be injected (500 mg/kg). Ticarcillin is approximately twice as active as carbenicillin against Pseudomonas aeruginosa. The acylureido penicillins, mezlocillin and piperacillin, are 8-10 times more active than carbenicillin against P. aeruginosa, and also have activity against K. pneumoniae, S. marcescens, H. influenzae, and Neisseria. Temocillin, with a 6a-methoxy group,

TABLE 25.2 Penicillins

OH H

-OCOC(CH3)3 -OCOOC2H5

Ampicillin

Amoxicillin

Picampicillin

Bacampicillin Talampicillin

Azlocillin Piperacillin o r

has improved resistance, against b-lactamase. It is quite active against enterococci, but not against

Pseudomonas.

Mecillinam is not an N-acyl derivative of 6-APA but is a semisynthetic 6-amidinopenicillanic acid (Table 25.3). It is very active against some Gram-negative bacteria like E. coli, Klebsiella, and Serratia but not against Pseudomonas, Proteus, and Haemophilus. For oral administration, the prodrugs pivmecillinam and bacmecillinam can be used.

25.2.1.3 Cephalosporins

In 1948, Brotzu at the University of Cagliari in Sardinia discovered antibacterial activity in the culture medium of a microorganism, Cephalosporium acremonium, collected at the sewage outlet of the town in the sea. This strain was sent to the laboratory of Professor Florey in Oxford. from its culture medium, Abraham et al. isolated in 1951, cephalosporin P, a steroid antibiotic related to fusidic acid, and in 1954 cephalosporin N, later called penicillin N, which is a penicillin with D-a-aminoadipic acid as side chain. When this penicillin was inactivated by acid, still another antibiotic, called cepha-losporin C was isolated (Abraham and Newton, 1955). They were able to determine its structure in 1961 and they found that it also had a D-a-aminoadipic acid side chain but a b-lactam-hydrothiazine ring system. The fact that this product was acid stable and resistant to penicillinase indicated that it could be the starting point of a new group of important antibiotics. The side chain, however, was

TABLE 25.3 Penicillins och3

V-chs c>chs

I ch3

COONa

COONa o

-CHI

COONa

Carbenicillin

Ticarcillin

Temocillin

COOR

Cyclacillin

Pivmecillinam Bacmecillinam Mecillinam

responsible for a rather low activity and had to be replaced by other groups. No microorganism was found that could enzymatically remove the side chain, but this transformation could be performed by chemical means. The first two cephalosporins introduced in the clinic, cefalothin (1962) and cefa-loridine (1964), have a thienylacetic acid side chain. Of the thousands of cephalosporins that were synthesized, only a few reached the clinic (Table 25.4).

They are often classified in generations. This classification is more or less related to the year of introduction (I:1962-1971, II:1974-1977, III:1976-1980) and their properties. One should not consider that cephalosporins of the third generation are superior in all respects to those of the first one.

Generation I cephalosporins are used mainly in infections with Gram-positive bacteria. The II generation is also active against certain Gram-negative bacteria like Neisseria and Haemophilus. The introduction of heterocyclic rings in C3 is responsible for the metabolic stability of these cepha-losporins. However, some structures like the tetrazolthiomethyl group (in cefamandole, cefotetan) are responsible for a disulfiram effect and prolonged blood coagulation. These side effects influence unfavorably their clinical application.

TABLE 25.4 Cephalosporins r1 — c—nh r1 — c—nh cooh

oh och3

oh ch

cooh

-ch3

cooh

Cephalosporin C

Cefalotin

Cefazolin

Cefaloridine

Cefamandole

Cefuroxime

Cefonicid

Ceforanide

In 1971 b-lactams with a methoxy group on C7, cephamycins, were discovered in certain Streptomyces strains. This substituent is responsible for a greater resistance to b-lactamase (Table 25.5).

In the Takeda laboratories, it was discovered in 1977 that reaction of a chloracetylcephalosporin with thiourea leads to a product with an aminothiazolylacetic acid side chain, e.g., cefotiam, which had a very good activity especially against Gram-negative bacteria. On the other hand, the presence of an oxime group in nocardicin was the guide for the introduction of this group in cefu-roxime, which has a good resistance against b-lactamase. These two observations were combined in the preparation of cefotaxime. A whole series of cephalosporins with an aminothiazol side chain are now available (Table 25.6).

TABLE 25.5 Cephalosporins

OCH3

H2N—CO NaOOC

COOH

COOH

Cephamycin C

Cefoxitin

Cefotetan

TABLE 25.6 Cephalosporins a.

COOH

COOH

Cefotaxime

Ceftazidime

Ceftriaxone

Cefepim

Cefpirom

TABLE 25.7 Cephalosporins

cooh R2

cooh R2

Cefaloglycin

Cefalexin

Cefradine

Cefadroxil n Cefatrizine

Cefixime

Cefaclor nh, o n r o

The blood concentrations obtained after oral administration of the above described cepha-losporins are low. Cefaloglycin, the first cephalosporin that gave moderate blood levels after oral administration, has a phenylglycine side chain. This group or a related group is present in several oral cephalosporins. Cefalexin, which is the most widely used product, has an activity that is much lower than that of the injectable compounds. Attempts to increase the activity have not been very successful, but cefaclor and cefixime are somewhat superior (Table 25.7).

25.2.1.4 Nontraditional b-Lactam Antibiotics

25.2.1.4.1 Clavulanic Acid and Sulbactam

Clavulanic acid discovered in the Beecham laboratories in 1976 is a b-lactam produced by the same Actinomycete Streptomyces clavuligerus that produces cephamycin C. Its in vitro activity is low, but it is a potent inhibitor of many b-lactamases. It is combined with conventional b-lactams like amoxicillin and ticarcillin. It potentiates the actions of these antibiotics against b-lactamase

16 r

Penicillin

Wchs

16 I

COOH H

6-Aminopenicillanic acid 6-APA

h h h2n

COOH

7-Aminopenicillanic acid 7-APA

Penam

Penem

Carbapenem X

Azetidin-2-one ß-lactam

Clavam

RCONH

O so3h

Monobactam

FIGURE 25.1 b-Lactam structures.

producing bacteria. Penicillanic acid sulfone (sulbactam) and its prodrug pivsulbactam are also inhibitors of b-lactamase and are combined with ampicillin (Figure 25.2).

25.2.1.4.2 Carbapenems

A new b-lactam antibiotic, thienamycin, was obtained in 1978 from cultures of Streptomyces cattleya by researchers at Merck Sharp and Dohme. The bicyclic system, with a double bond between C2 and C3, is called 2-carbapenem (Figure 25.1). Several related products were discovered, e.g., epithienamycins (isomers in hydroxyethyl side chain). Thienamycin is not stable and is used as the N-formimidoyl derivative (imipenem). Thienamycin and imipenem are prepared by total synthesis. Imipenem and meropenem are broad-spectrum antibiotics. Another problem was discovered during clinical studies, i.e., the cleavage of thienamycin by a dehydropeptidase present in the kidney. For that reason, imipenem is associated with an inhibitor of that enzyme, cilastatin. Imipenem exhibits a broad spectrum of activity and is resistant to most b-lactamases (Figure 25.2).

25.2.1.4.3 Penems

In 1977, Woodward described the first synthesis of a penem. This penem, which had a phenacetyl-amido side chain like penicillin, had a rather low activity. The introduction of hydroxyethyl groups in C6, like in the carbapenems, improved the potency significantly.

COOH Thienamycin

COOH

COOH Thienamycin

OH Clavulanic acid

CONMe

Meropenem

CONMe

ICOOH H

Meropenem

COO Imipenem

Sulbactam

I jch3

COO Imipenem

Pivsulbactam

CH2 COONa

COONa Cilastatin (Na)

FIGURE 25.2 Nonclassical ß-lactams.

25.2.1.4.4 Monobactams

The term monobactam was coined by Sykes et al. (1981) to describe a novel group of bacterially produced monocyclic ß-lactams. A product SQ26.180 was isolated by Sykes from Chromobacterium violaceum and a related product, sulfazecin was discovered in Japan by Imada et al. (1981) in cultures of Gluconobacter sp., and Pseudomonas sp. The activity was improved by modification of the amide side chain. The introduction of the cefotaxime side chain gave an interesting new drug, aztreonam. It is very active against Gram-negative bacteria, including Pseudomonas aeruginosa (Figure 25.3).

25.2.2 Bacitracin

Bacitracin is produced by a strain of Bacillus subtilis and was isolated in 1945 from the wound of a patient called Tracy. The same product was discovered in Oxford in 1949, in the culture of Bacillus licheniformis A5, and was called Ayfivin. Studies by Abraham et al. and by Craig et al. in New York revealed that bacitracin (= ayfivin) was a mixture and it had a peptide structure. The structure of the main component is given in Figure 25.3.

Aztreonam

L-Leu

D-Phe

D-Glu

L-Ileu

D-Orn

Bacitracin A

FIGURE 25.3 Aztreonam and bacitracin A.

H3C COOH

25.2.3 Vancomycin and Related Products

Vancomycin was isolated by Williams in 1956 in the Lilly laboratories from a culture of Streptomyces orientalis. Its very complex structure was studied mainly by D.H. Williams in Cambridge.

Vancomycin is active only against Gram-positive microorganisms, mainly staphylococci and streptococci, and also against Clostridium difficile. It is a quite toxic product. As intramuscular injections cause pain and necrosis, the administration has to be intravenous. Febrile reactions, thrombophlebitis, ototoxicity, and nephrotoxicity are the other side effects. For these reasons, its use is limited to infections due to resistant staphylococci and streptococci, mainly in patients treated with immunosuppressants. Oral application may be used in pseudomembranous colitis, due to Clostridium difficile, which may occur after the administration of antibiotics like lincomycin and clindamycin. Teicoplanin, also named teichomycin, was isolated in 1978 from Actinoplanes teicho-mycelicus. Its structure and activity spectrum are related to that of vancomycin.

25.3 ANTIBIOTICS AFFECTING THE CYTOPLASMIC MEMBRANE 25.3.1 Antibiotics that Disorganize Membrane Structure 25.3.1.1 Tyrothricin

Tyrothricin was isolated from Bacillus brevis by R. Dubos in 1939. Because of its toxicity, e.g., lysis of erythrocytes, its application has always been limited to external use like troches for throat infections, nose drops, etc.

It is a mixture of a neutral acetone-ether soluble part (about 20%) called gramicidin and a solvent insoluble part, tyrocidine HCl. Tyrocidine is a cyclic peptide. Gramicidin is a mixture of neutral linear peptides. It is neutral because the N-terminal amino group is formylated and because the carboxy group is linked to ethanolamine (Figure 25.4).

AZI-Gly-L-Ala-D-Leu-L-Ala-D-Val-L-Val-D-Val-L-Try-D-Leu-AzII-D-Leu-L-Try-D-Leu-L-Try-NH

AZII

Valine gramicidin A

L-Val L-Val

L-Try L-Phe

Isoleucine gramicidin A

L-Val t

L-Tyr

L-Ileu L-Ileu HCl L-Orn

L-Try L-Phe

Glu -L-Asp

NH2 NH2

L-Pro

Glu -L-Asp

NH2 NH2

Y

Z

Tyrocidine A:

D-Phe

D-Phe

Tyrocidine B:

L-Try

D-Phe

FIGURE 25.4 Tyrothricin components.

FIGURE 25.4 Tyrothricin components.

25.3.1.2 Polymyxin

Polymyxin was discovered in 1947 almost simultaneously in three different laboratories in the United States and England. It has been shown that Bacillus polymyxa and B. aerosporus were identical species and an agreement was reached on one name: polymyxin. Several polymyxins A, B, C, D, and E were isolated, but polymyxin B, which seemed to be the least toxic, was introduced in medicine. A peptide antibiotic, colistin, isolated in 1950 in Japan from B. colistinus was shown to be identical with polymyxin E. Both polymyxin B and E contain several components differing in the structure of the fatty acid. The most important component is Bj (or E1), with some 15%-25% of B2 (or E2). Polymyxin and colistin have five free amino groups and are used as the sulfate salt. A sulfomethyl derivative, obtained by reaction with formaldehyde and bisulphite, and which is less toxic, is used for injection. These peptides have been used for the treatment of infections by Gramnegative bacteria like Pseudomonas, but this treatment is seldom used now, because less toxic antibiotics are available. Because the

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