ANTIVIRAL DRUGS

1

ANTIVIRAL DRUGS

Přemysl Mladěnka & Eduard Jirkovský

Department of Pharmacology and Toxicology, Faculty of Pharmacy, Charles University

Basic definition:

 drugs used specifically for treatment of viral infections (virosis)

Pathophysiology of virosis:

 Obligate intracellular pathogens

 Structure of viral particles (“virion”) consists: (Fig. 1)

 Double- or single-stranded RNA or DNA

 A protein coat (“capsid”), for some viruses also a lipid envelope over the capsid

 Besides nucleic acids (NA), specific enzymes could be present inside the virus depending on the virus type (ribosomes, reverse

transcriptases, integrases, proteases etc.)

 Viruses are unable to autonomously carry out any metabolic processes – fully dependent on a host cell

 Some viruses destroy infected host cells, others just implement their NA into host genome

2

Fig.1. Schematic overview of viral particle (virion).

Note: Many of viruses has even simpler structure.

Classification of viruses:

1. DNA-viruses

 viral DNA is transcribed into mRNA by host polymerases and replicated by viral DNA-polymerase (Fig. 2)

3

Fig. 2. Schematic overview of majority of DNA viruses.

Note: Some DNA viruses (HBV) use reverse transcriptase.

Representatives:

 poxviridae (variola virus causing smallpox)

 herpesviridae (herpes simplex virus 1 and 2 (HSV), varicella zoster virus, Epstein-Barr virus, cytomegaloviruses)

 adenoviridae (causing mainly common cold and infection of upper respiratory tract)

4

 hepadnaviridae (hepatitis B virus (HBV))

 papillomaviridae (human papilloma virus (HPV)) a polyomaviruses (BK virus and JC virus affecting immunocompromised patients)

2. RNA-viruses

– Depending on their classification, protein translation is performed directly from viral RNA or mRNA must first be synthetized (Fig. 3)

–

Obr. 3. Schematic overview of replication of RNA viruses. A: (-)-RNA viruses, B: (+)-RNA viruses a C: Retroviruses (HIV).

5

Figure retrieved from https://media.nature.com/lw685/nature- assets/nrmicro/journal/v4/n5/images/nrmicro1389-i1.jpg.

representatives:

 picornaviridae (poliovirus, hepatitis A virus (HAV), Coxsackievirus and echovirus, rhinoviruses)

 togaviridae (alphavirus causing tick-borne encephalitis, rubivirus causing rubella, hepatitis C virus (HCV))

 rhabdoviridae (lyssavirus causing rabies)

 orhtomyxoviridae (influenza viruses type A-C causing flu in humans and other species – A/H5N1-avian flu, A/H1N1 swine flu)

 paramyxoviridae (mumps rubulavirus, measles morbillivirus, human respiratory syncytial virus (RSV))

 coronaviridae (generally causing common cold and infection of upper respiratory tract; betacoronaviruses causing severe acute respiratory syndrome (SARS) and middle east respiratory syndrome (MERS))

 retroviridae (HIV, Fig. 3B and Fig. 4)

 contains reverse transcriptase that transcribes RNA into DNA and subsequently integrase incorporates it into host DNA

 provirus - a copy of viral DNA incorporated into host genome

6

Fig. 4. Structure of HIV virus.

Retrieved from Wikimedia.

Simplified general life cycles of viruses (Fig. 5):

 1. Entry of virus into host cell which comprises attachment of virus to specific receptors on host cell surface, and penetration into the cell by the process of endocytosis or membrane fusion.

 2. „Uncoating“ – degradation of viral capsid by viral or host enzymes and releasing of the viral NA

7

 3. Transcription of viral mRNA (Fig. 5 - 3A) and replication viral DNA/RNA (3B), event. Integration of viral NA into host DNA (3C)

 4. Translation of viral proteins

 5. Posttranslational modification and virion formation from individual components

 6. Virion release from host cell

Fig. 5. General schema of virus replication.

For description of separate phases see text above.

8

Therapy of viroses

 is generally difficult, currently causal treatment does not exist for many viroses

 the best therapy of choice is prevention, esp. in the form of active immunization by vaccination or passive immunization (a person is given someone else’s antibody)

 ACTIVE IMMUNIZATION

o mandatory (CZ) vaccinations against selected viroses:

 polyomyelitis (Polio Sabin^®, Imovax Polio^®)

 measles, mumps and rubella (Trivivac-Sevac^®)

 HBV ^{(Engerix®, Hbvaxpro ®)}

o Obligatory ones:

 HAV (Havrix^®)

 flu ^{(Begrivax®, Fluarix®, Influvac®, Vaxigrip®;Fluad®;Inflexal®;Optaflu®)}

 HPV positive cervical cancer ^{(Silgard®, Cervarix®)}

 tick-borne encephalitis ^(Ecepur®, FSME Immun^®)

 rabies ^{(Rabipur®, Verorab®)}

 rotaviruses (Rotarix^®;Rotateq^®)

 varicella (chickenpox) and herpes zoster (shingles) (monokomp.

Varilrix^®, for prevention of herpes zoster Zostavax^®)

 THE PITFALLS OF ANTIVIRAL TREATMENT

9

 Strong antiviral effects in vitro but in vivo troublesome AE

 Narrow spectrum of antiviral activity

 Fast resistance development – antiviral combination is necessary to treat severe infections (hepatitis C or HIV)

 Generally, antiviral drugs block replication of viruses  risk of relapse. Effective host immune defence is necessary to cure of the viroses.

10

ANTIVIRAL DRUGS

Phase^* Part of life cycle Mechanism Antiviral spectrum

1 attachment and penetration

passive immunization

block of penetration of HIV virus

individual only AIDS

2 uncoating Prevention of release of viral RNA into cytoplasm Only influenza A (obsolete) 3 Nucleic acid

synthesis

Inhibition of (DNA-dependent)-DNA polymerase Inhibition of RNA-polymerase, incl. whole

replication complex

Inhibition of reverse transcriptase Inhibition of integrase

Mainly herpesviruses hepatitis C

AIDS, hepatitis B AIDS

4 Translation Inhibition of translation process by interferons Antisense nucleotides

broad

currently only cytomegaloviruses 5 Posttranslational

modification

Inhibition of proteases AIDS, hepatitis C

6 Release of virions Inhibition of neuraminidase Only influenza A and B

* see Fig. 5 for particular phases

P^HASE 1:INHIBITION OF PENETRATION OF VIRUS TO HOST CELL Human -globulins

 A fraction of human plasma rich for majority if IgG antibodies (i.e. passive immunization)

 Mainly use for prophylaxis propose

 Available also specific immunoglobulins against specific virus

 AE: hypersensitivity reaction, inc. anaphylaxis maraviroc ^{(Celsentri®)}

 Prevent penetration of HIV viruses¹ using a chemokine receptors CCR5 on host surface

 2x daily, p.o.

Other drugs²

P^HASE 2:INHIBITION OF VIRION UNCOATING

amantadine (Viregyt-K cps. ^®)³

 Nowadays used rarely

 Effective selectively against influenza virus A

 Chemical structure is close to tromantadin (Viru-Merz gel^®) – but this drug has different antiviral spectrum and probalby mechanism of action

1enfuvirtide (Fuzeon^®) also belongs into this group – it blocks fusion of HIV-1 virus with host cell membrane;

unfavorable PK, must be administered twice daily s.c.

2 docosanol (Erazaban crm®) – herpes labialis, palivizumab (Synagis^®) – IgG1 monoclonal antibody against RSV surface antigen

3rimantadine (not registered yet) is 4-10x effective than amantadine

P^HASE 3:INHIBITION OF VIRAL DNA-^DEPENDENT-DNA

POLYMERASE

Mechanism of action:

 These are analogues of natural nucleosides – fake base⁴

 All are prodrugs – they need to be phosphorylated inside cells

 Phosphorylation to monophosphate form could be realize either by viral or host enzymes, subsequent phosphorylation to triphosphate form is performed by host enzyme

 Incorporation of triphosphorylated fake nucleotides (active form of the drug) into the chain of viral DNA terminates its elongation and caused also formation of inactive complex with viral DNA-polymerase

 Majority of these drugs possess only low affinity to human DNA- polymerases

Pharmacokinetics:

 Absorption significantly varies between representatives, derivatization of parent compound also employed (L-valine prodrug of aciclovir)

 Excretion via glomerular filtration and tubular secretion

 Plasma half-life of these drug is not suitable indicator for prediction of therapeutic efficiency because more important is half-life of drug’s triphosphate inside the cell

Purines nucleotides analogues

4 Non-nucleoside representatives also exist (e.g. foscarnet, is not any more in clinical practice in CZ)

aciclovir (cps: Herpesin^® a Zovirax^®, řada lokálních preparátů)

valaciclovir (Valtrex^®, Valaciclovir Mylan^®,Valaciclovir+Pharma^®)

 A prodrug, ester of L-valine with aciclovir

 Aciclovir is analogue of guanosine with high specificity for herpes viruses

 The specificity is based on selective phosphorylation of parent aciclovir only by viral enzymes (Fig. 6)





 Fig. 6. Schema of bioactivation of aciclovir in human body.

VE – viral enzyme, HE – human enzyme.

Adverse effects:

o Regarding the specificity, treatment with aciclovir is associated only with minimal toxicity (nausea, headache, diarrhea)

o In higher doses may cause CNS adverse effects (somnolence, tremor, hallucination) and reversible nephrotoxicity

ganciclovir (Cymevene inf^®, Virgan oph gel^®)

valganciclovir (tbl: Valcyte^®, Valdamin^®, Valganciclovir Mylan^®, Valganciclovir Teva^®, Virexan^®)

 L-valylester of ganciclovir

 In action similar to aciclovir but treatment is associated with higher severity of adverse effects

 Ganciclovir lack the specificity of aciclovir, and its active form is incorporated also into host DNA

Antiviral spectrum:

 effective against all herpesviruses, mainly against cytomegaloviruses AE of ganciclovir:

 May be serious and life-threatening – induces bone marrow depression (neutropenia, thrombocytopenia)

 A potential carcinogen and teratogen

penciclovir (crm. Vectavir^®) – analogue of guanosine similar to aciclovir, only for local administration

cidofovir (Vistide inf.sol.^®) – has broad antiviral spectrum, however also higher toxicity

P^HASE 3:INHIBITION OF REVERSE TRANSCRIPTASE (^I.^E.RNA-

DEPENDENT-DNA POLYMERASE)–NUCLEOSIDES

 Also called antiretroviral nucleoside analogues

 Similar mechanism of action as inhibitors of DNA-dependent-DNA polymerase

 prodrugs, they are metabolizes by human enzymes to their active form → block of reverse transcriptase (RT) of HIV viruses or hepatitis B virus Pharmacokinetics:

 good absorption

 half-life of triphosphates of majority of representatives sufficient enough to enable dosing once or twice daily, p.o.

 elimination through kidney by active tubular secretion and glomerular filtration

Adverse effects:

 in therapeutical doses, these drugs have more higher affinity to RT than to DNA-polymerase in host cells

 however, some of them are enable to inhibit human mitochondrial DNA- polymerase  specific AE

 sporadic risk of lactate acidosis and hepatic steatosis (based on inhibition of host polymerases) – increase risk with concomitant liver or pancreas disease

 sporadic lipodystrophy syndrome – due to HIV itself or the therapy?

Drugs for treatment of both HIV and hepatitis B:

lamivudine (deoxythiacytidine) a tenofovir

 both drugs are well tolerated by patients

Drugs for treatment of HIV only⁵

zidovudine (azidothymidine), abacavir, emtricitabine

Drugs for treatment of hepatitis B only:

adefovir-dipivoxil (Hepsera^®), entecavir (Baraclude^®),

telbivudine (Sebivo^®)

 L-form of thymidine

PHASE 3:INHIBITION OF REVERSE TRANSCRIPTASE-NON- NUCLEOSIDES

 Used only for treatment of HIV-1 virus

 All are non-competitive inhibitors of RT of HIV-1

 1^st generation:nevirapine, efavirenz

o fast development of resistance

 2^nd generation:etravirine, rilpivirine

o Lower risk of the resistance development Pharmacokinetics:

 Good absorption

 Metabolized via hepatic CYP450

5 List of all registered medicinal products in CZ (08/2018) from this group is available in Appendix I.

Adverse effects:

 Urticaria, elevation of transaminases

 CNS disturbances (sleep disturbances, mood swings and impaired concentration etc.) after treatment with efavirenz and rilpivirine Interaction:

 Many of representatives are medium-strong inductors of CYP450 ^ increased potential to pharmacotherapeutic interactions

P^HASE 3:INHIBITION OF VIRAL RNA-^POLYMERASE, ^EV. ^NON-

STRUCTURAL PROTEIN 5A(NS5A)⁶

 Inhibitors of RNA-polymerase (suffix „–buvir“) may be either of nucleoside (sofosbuvir) or non-nucleoside structure (dasabuvir)

 Inhibitors of “helper” non-structural protein 5A (NS5A) (suffix „–asvir“), e.g. daclatasvir

P^HASE 3:INHIBITION OF HIV ^INTEGRASE

raltegravir⁷

 2x daily, p.o., very good absorption

P^HASE 3:O^{THER DRUGS}

ribavirin (Fig. 7, also as tribavirin; tbl: ^{Copegus®,Rebetol®}, Ribavirin Mylan^®)

 analogue of guanosine, precise mechanism of antiviral action is unknown

6 List of all registered medicinal products in CZ (08/2018) from this group is available in Appendix II.

7 Newer drugs elvitegravir and dolutegravir – for more info see Appendix I.

Fig. 7: Chemical structure of ribavirin

Antiviral spectrum:

 broad, contains DNA and RNA viruses (e.g. ortho-, paramyxoviridae, flaviviruses)

Adverse effects:

 among others lesser serious, risk of teratogenicity and anaemia P^HASE 4:INTERACTION WITH TRANSLATIONAL AND

POSTTRANSLATIONAL PROCESSES

interferons (INF)

 are synthesized by many cells as a response to viral infection

 belong to cytokines with antiviral, antiproliferative and immunomodulatory effects

 high antiviral efficacy in IFN α and β, IFN γ has lower antiviral effect

 broad antiviral spectrum Mechanisms of action:

 inhibition of transcription

 inhibition of translation (probably the leading effect)

 inhibition of glycosylation of proteins Pharmacokinetics:

 only for parenteral administration (s.c., i.m.) Adverse effects:

 flu-like symptoms

 myelosuppression  granulo- and trombocytopenia, neurotoxicity

 AE of IFN may cause symptoms generally similar to the viral infection Indications:

 IFN-: chronic hepatitis B a C⁸, INF--2a (Roferon-A inj ^®), INF--2b (Introna inj^®)

and pegylated interferon--2a ^(Pegasys®)

 IFN-^: see multiple sclerosis

fomivirsen – antisense nucleotide for treatment of cytomegaloviral infections, not-marketed in CZ

P^HASE 5:INHIBITION OF PROTEASES

 Primary was inhibition of viral proteases used only for treatment of HIV, currently is used also in treatment of hepatitis C

8 Currently, clinical use of INFα in treatment of hepatitis C lowers thanks to novel more effective medicaments.

1. Treatment of HIV

 Selectively block HIV-1 and HIV-2 proteases → cause production of immature virions

 Resistance against one inhibitor is often not cross-linked to others.

Pharmacokinetics:

 p.o. administration

 all are substrates and often strong inhibitors of CYP 3A4 Representatives:

 currently 8 drugs are registered for treatment of HIV⁹, e.g. ritonavir Adverse effects:

 dominant are GIT symptoms (nausea, diarrhoea, vomiting, dysorexia)

 lipodystrophy

 hepatotoxicity (elevation of transaminases)

 haematopoiesis depression, neuropathy Interaction:

 inhibition of CYP3A4!

 Combination with drugs induce activity of CYP 3A4  increase of ritonavir clearance

2. Treatment of hepatitis C

 Inhibition of NS3/4A serine proteases of hepatitis C virus

9 See Appendix I

 Introduction of the first two representatives into clinics in 2011¹⁰ was a milestone in a treatment of hepatitis C

 Currently are in use representatives of 2^nd and 3^rd generation¹¹

 All these representatives have suffix „-previr“, e.g. voxilaprevir

PHASE 6:INHIBITION OF VIRIONS RELEASE

oseltamivir (Tamiflu^®) and zanamivir (Relenza plv.ad.inh.^®)

Mechanism of action:

 Inhibitors of neuraminidase, effective in prevention and treatment of influenza A and B viruses

Pharmacokinetics:

 oseltamivir is administered p.o., zanamivir only for inhalation Adverse effects:

 oseltamivir: GIT and CNS symptoms

 zanamivir: respiratory symptoms

10boceprevir (Victrelis cps) a telaprevir (not registered yet)

11 Complete list of representatives in Appendix II

Appendix I: List of registered medicinal products for treatment of HIV infection.

Monocomponent medicinal products

nucleoside RT inhibitors

abacavir ^Ziagen®

lamivudine ^{Epivir®, 1LamivudinTeva®,}

1Zeffix^®

stavudine ^Zerit®

tenofovir^{2 Viread®, Ictady®,Tenofovir} disoproxil Mylan^®, Tenofovir disoproxil Mylan^®,Tenofovir disoproxil Sandoz^®,Tenofovir disoproxil Teva^®, ¹Vemlidy^®

zidovudine ^Retrovir®

non-nucleoside RT inhibitors

efavirenz ^Stocrin®, Efavirenz Teva^®

etravirine ^Intelence®

nevirapine ^Viramune®

rilpivirine ^Edurant®

inhibitors of HIV protease

atazanavir ^Reyataz®,+cobistat Evotaz^®

darunavir ^Prezista®

fosamprenavir ^Telzir®

indinavir ^Crixivane®

lopinavir (+ritonavir) ^Kaletra

ritonavir ^Norvir®

sachinavir ^Invirase®

tipranavir ^Aptivus®

dolutegravir ^Tivicay®

inhibitors of integrase

raltegravir ^Isentress®

inhibitors of HIV virus penetration into host cells

enfuvirtide ^{Fuzeon inj®}

maraviroc ^Celsentri®

Polycomponent medicinal products

2 nucleoside inhibitors of RT

lamuvidine + zidovudine ^Combivir®.

Lamivudin/zidovudinMylan^® tenofovir + emtricitabine ^{Truvada®, Descovy®, Ictastan®,}

Emtricitabine/tenofovir disoproxil Krka^® lamuvidine + zidovudine ^Combivir®.

Lamivudin/zidovudinMylan^® abacavir + lamivudine Abacavir/lamivudine Teva^®,

Iviverz^®,Kivexa^® 2 nucleoside

inhibitors of RT+ 1 non-nucleoside

emtricitabine + tenofovir + rilpivirine

Eviplera^®, Odefsey^®

2 nucleoside inhibitors of RT + inhibitor integrase (ev. a metabolic inhibitor)

emtricitabine + tenofovir + elvitegravir + cobicistat

Stribild^®, Genvoya^®

abacavir + lamivudine + dolutegravir

Triumeq^®

3 nucleoside inhibitors of RT

abacavir + lamivudine + zidovudine

Trizivir^®

1 approved only for hepatitis B treatment, ² approvedfor hepatitis B and also HIV treatment RT- reverse transcriptase

Appendix II. Lis of registered medicinal products directly effective against hepatitis C.

Monocomponent medicinal products

Inhibitors of proteases

boceprevir asunaprevir

Victrelis^® Sunvepra^®

inhibitors of NS5A daclatasvir ^Daklinza®

inhibitors of RNA- polymerase

sofosbuvir dasabuvir

Sovaldi^® Exviera^® Polycomponent

medicinal products

inhibitors of RNA- polymerase and non- structural protein NS5A

sofosbuvir + ledipasvir sofosbuvir + velpatasvir

Havroni^® Epclusa^®

inhibitors of proteases and RNA- polymerase

grazoprevir + elbasvir paritaprevir + ombitasvir (+ritonavir)

Zepatier^®

Viekirax^® inhibitors of

proteases, RNA- polymerase and non- structural protein NS5A

voxilaprevir + sofosbuvir + velpatasvir

Vosevi^®

References:

 Balzarini J. Current status of the non-nucleoside reverse transcriptase inhibitors of human immunodeficiency virus type 1. Curr Top Med Chem. 2004;4(9):921-44.

 Brayfield A et al. Martindale The complete drug reference. 38th edition. Pharmaceutical press: London, 2014

 Brunton L, Chabner B & Knollman B. Goodman & Gilman's The Pharmacological Basis of Therapeutics, 12^th edition. Mc Graw- Hill: New York, 2011.

 Bryan-Marrugo OL, Ramos-Jiménez J, Barrera-Saldaña H, Rojas-Martínez A, Vidaltamayo R & Rivas-Estilla AM. History and progress of antiviral drugs: From acyclovir to direct-acting antiviral agents (DAAs) for Hepatitis C. Medicina Universitaria 2015;17(68):165–74.

 De Clercq E, Li G. Approved Antiviral Drugs over the Past 50 Years. Clin Microbiol Rev. 2016;29(3):695-747.

 Flexner C. HIV-protease inhibitors. N Engl J Med. 1998;338(18):1281-92.

 Malaty LI, Kuper JJ. Drug interactions of HIV protease inhibitors. Drug Saf. 1999;20(2):147-69.

 Matrosovich M, Herrler G, Klenk HD. Sialic acid receptor of viruses. Top Curr Chem 2015;367:1-28.

 McCauley JA, Rudd MT. Hepatitis C virus NS3/4a protease inhibitors. Curr Opin Pharmacol. 2016;30:84-92.

 Rosenthal KS, Sokol MS, Ingram RL, Subramanian R, Fort RC. Tromantadine: inhibitor of early and late events in herpes simplex virus replication. Antimicrob Agents Chemother. 1982;22(6):1031-6.

 Wikimedia - https://commons.wikimedia.org/wiki/ File:Hiv_gross.png , 10.9.2018