BIOC20 Lecture 12 - Antiviral Vaccines PDF

Lecture 12 Antiviral Vaccines History Categories Immune response Adjuvants New technologies Safety and Ethics Antiviral Drugs / Drugs targeting: Attachment / entry Uncoating Nucleic acid synthesis Integrase, proteases and n...

Lecture 12 Antiviral Vaccines History Categories Immune response Adjuvants New technologies Safety and Ethics Antiviral Drugs / Drugs targeting: Attachment / entry Uncoating Nucleic acid synthesis Integrase, proteases and neuraminidase Future 1  Antiviral Vaccines - History Vaccination is the most effective means to prevent viral infections Additionally: Serious adverse effects must be investigated, however they are very rare. If a vaccine is not 100% safe / effective, it raises ethical concerns 2  Antiviral Vaccines - History Early vaccine technology was crude but effective Crude vaccines from ‘natural products’ worked well but due to technological limistations they resulted in serious side effects → convincing general public to vaccinate was often established by fear of ongoing epidemics, some with severe visible effects. 3  Antiviral Vaccines - History Use of primary cell culture and immortalized cell lines accelerated the development of vaccines in the 1950-1960’s. Three major technological leaps: 1. Serial passage of human virus in animals 2. Virus growth in sterile conditions and embryonated chicken eggs 3. Growth of virus in vertebrate cells cultured in vitro 4  Antiviral Vaccines - History Example: Production of vaccines against avian influenza strains has been problematic: A major panzootic of H5N1 avian influenza occurred between 1997-2009 Panzootic: widespread increase in infectious disease in animals over a large geographic range (i.e. epidemic in animals) Highly pathogenic in birds, rare infection in humans Mutation of that virus could have lead to a major pandemic in humans with mortality rates of 33-70% Serious technical problems for vaccine production: Virus was so pathogenic that chick embryos were killed before sufficient vaccine virus amounts could be produced. 5  Antiviral Vaccines – Classical Categories Current antiviral vaccines can be grouped into different categories: Live wild-type viruses Simple and very effective Viruses may replicate poorly in non-natural host but share immunogenic determinants (ability to induce an immune response) with related target viruses (e.g. vaccinia virus) Live attenuated viruses Generally, most effective Serial passage and in vitro cultivation of viruses reduces their pathogenicity 6  Antiviral Vaccines – Classical Categories Serial passaging Virus grown in vitro → over time results in mutations that may make it less pathogenic →→→→ can replicate in humans and can give immune protection inducing little or no disease Some techniques modify the virus such that it has restricted growth at 10yrs) Simple delivery, some can be taken orally or nasally Are expensive and unstable, requiring attention to refrigeration (cold supply chain) → must be maintained from manufacture to administration (sensitive to heat AND freezing) May be subject to inactivation by maternal antibodies when administered in early childhood Very rarely causes serious illness and death 10  Antiviral Vaccines – Advantages and Disadvantages Inactivated (whole virus) vaccines: Generates immune response to a wide range of viral antigens Generally less sensitive to storage issues Safer for immunocompromised individuals Expensive production and requires manipulation of virulent pathogens Less successful in inducing a robust immune response (Why?) → requires multiple doses, boosters Must be injected and may require the addition of an adjuvant to elicit a strong immune response 11  Antiviral Vaccines – Advantages and Disadvantages Subunit vaccines: Simplest and least expensive to make Very stable Safer in immunocompromised patients Avoids delivery of irrelevant or potentially immunopathologic viral proteins causing adverse effects Multiple doses and boosters are often required Typically requires an adjuvant 12  Antiviral Vaccines – Newer Categories Chimeric Vaccines Contain live, genetically modified (non-pathogenic) organisms that express viral proteins of interest (chimera = more than one genotype) Can deliver antigens to desired locations (gut mucosa, dendritic cells, etc.) RNA and DNA Vaccines Based on mRNA and/or DNA plasmids encoding one or more viral proteins in a form that can be expressed in host cells (transient expression) e.g. COVID-19 vaccines from Pfizer and Moderna are mRNA vaccines! Nanoparticle Vaccines (includes VLPs) Synthetic nanoparticles (e.g. liposomes) displaying multiple copies of viral antigens and can be combined with adjuvants e.g. Novavax COVID-19 vaccine! 13  Antiviral Vaccines – Newer Categories Recombinant vaccines use genomic fragments that encode for immunogenic viral proteins → engineered into a non-pathogenic virus (or bacteria e.g. lactobacillus) or into a plasmid → expression in host → immune response Can be used to make VLPs, nanoparticles and multivalent peptide vaccines (hundreds of epitopes) etc. Nanoparticles Additional molecules e.g. antigens against cellular pathogen recognition receptors can be incorporated to elicit a strong immune response Fig. 35.2, modified 14  Antiviral Vaccines – Newer Categories Example: Virus vector DNA (e.g. AstraZeneca) and mRNA vaccines (Pfizer, Moderna) for SARS- CoV2 15 Antiviral Vaccines – SARS-CoV2 vaccines 16 Antiviral Vaccines – SARS-CoV2 vaccines 17  Antiviral Vaccines – Immune Response Two main mechanisms: 1. Cellular immune responses mediate recovery from a majority of viral illnesses: Killing of infected cells via activated cytotoxic lymphocytes (CTLs) is main mechanism to eliminate infection 2. Neutralizing antibodies (NAbs) can confer protection to most viral pathogens: NAbs limit infection/re-infection and provide cross protection to other similar viruses Memory T and B cells support long-term production of Nabs Passive antibody transfer from mother to infant protects babies Antibodies protect against infection, cellular immunity helps clearance/recovery of infection18  Antiviral Vaccines – Immune Response Generation of antibodies may not be sufficient to control some viruses: Viruses that establish latent infections (e.g. herpesvirus) Viruses with multiple serotypes (variants) and limited cross-neutralization Rapidly mutating viruses (e.g. influenza A virus) Viruses that establish chronic infections (e.g. HIV) Other approaches are being developed e.g. enhancing cellular immunity (does not require antibodies) There is currently no ‘universal fit’ for the development of antiviral vaccines Urgent need for vaccines against viruses that cause significant illness e.g. RSV, HIV, Dengue etc. 19  Antiviral Vaccines – Adjuvants Adjuvants play an important role in vaccination and new adjuvants are under development: Can be added to boost the immune response to produce more antibodies and longer-lasting immunity, thus minimizing the dose of antigen needed Stimulates secretion of cytokines that favour production of antibodies Adjuvants are generally not required for live attenuated virus vaccines because they replicate in the body, inducing both cellular and humoral responses like a natural infection e.g. Aluminum salts preferentially stimulate helper T lymphocytes to secrete cytokines that favor production Inactivated (whole virus) and Subunit vaccines cannot replicate of antibodies → less efficient induction of immunity → often need stronger adjuvants 20  Antiviral Vaccines – New Technologies Extensive research on newer technologies, delivery systems to improve vaccinations: 21  Antiviral Vaccines – New Technologies Ongoing research also involves better delivery technologies / strategies 22  Antiviral Vaccines – Safety Current vaccines are the safest ever marketed, but nothing can be 100% safe The history of vaccine development is plagued by several unfortunate incidents → due to incomplete knowledge on many immunological reactions, most adverse events are poorly understood or difficult to foresee New worldwide surveillance initiatives ensures safety and confidence 23  Antiviral Vaccines – Safety Vaccines create a dead end for the virus by blocking virus replication and spread in many ways: Limits mutations – harder to generate virus variants if the infection is not successful Reduces viral load and transmission Protects others with underlying conditions or unvaccinated (e.g. herd immunity) Primes immune system for re-infection → less severe disease Can lead to eradication and global protection 24  Antiviral Vaccines – Herd Immunity Herd Immunity: Spread of viral pathogen is limited due to substantial vaccination in the population → protects small number of unvaccinated individuals or for whom vaccination has failed Need a threshold of ‘dead-end’ hosts Cannot reach the threshold if too many people choose not to vaccinate (increasing problem!) The vaccine coverage required for herd immunity is different for each virus and community → can change over time! 25  Antiviral Vaccines – Ethical Issues Individuals who refuse vaccination (personal choice, religious reasons, misinformation etc.; medical consent is required by law), managing their care, should they become infected and preventing further spread from them is controversial Should non-vaccinated be excluded from schools and workplaces? How do we protect and ensure inclusion of the vulnerable in our communities? Who bears the healthcare burden for life-long infection with vaccine preventable diseases? Who ensures those with vaccine-related adverse events are adequately compensated? Who enforces governments and industry to market the safest possible formulations for widespread administration? No single ‘correct’ answer. Need to have balanced responsibilities at all levels. 26  Antiviral Drugs The discovery and widespread use of antiviral compounds only began relatively recently e.g. first antibiotics developed in 1940’s and first antivirals in 1960’s Viruses grow within human cells (using host cell machinery) → fewer obvious targets → delayed antiviral discovery Broad spectrum antivirals are unlikely due to a large number of viruses → results in less pharmaceutical interest ($$$) Available vaccines also lessen the interest in designing new antivirals ($$$) 27  Antiviral Drugs How are antiviral drugs discovered / obtained? “Serendipity” – trying out compounds used for other purposes Chemical modification of known active compounds High throughput screening assays of many compounds Rational design, often with the aid of 3D structures of viral proteins AI tools are a new development → can potentially speed up the process significantly Antiviral drugs are useful for discoveries in basic research on viruses e.g. blocking steps in viral life cycle can reveal timing of particular events and identify roles of proteins Mutations that confer drug resistance can help identify specific roles for viral proteins28  Antiviral Drugs Antivirals are generally targeted to specific steps of virus replication: Fig. 36.1 29  Antiviral Drugs Therapeutic Index (TI): Ratio of the dose that exerts toxic effect in 50% of the population (TD50) to the dose that exerts a therapeutic or effective response in 50% of the population (ED50) TI = TD50 / ED50 What is the therapeutic index of a drug shown below? Would it be better to have high or low TI? 30  Antiviral Drugs – Attachment / Entry Drugs preventing attachment and entry of virions: Drugs can bind to virions directly Capsid-binding drugs block attachment and/or entry, and therefore all subsequent steps of replication as well E.g. Pleconaril blocks many picornaviruses, binding to the hydrophobic pocket in VP1 capsid protein, stabilizing capsid to prevent conformational change; Temsavir prevents conformation change in HIV gp120 and prevents attachment: Fig. 36.2 31  Antiviral Drugs – Attachment / Entry Drugs preventing attachment and entry of virions: Drugs can bind to virus receptors or co-receptors and prevent fusion E.g., Maraviroc binds to the cellular HIV-1 co-receptor CCR5, blocking attachment; Enfuvirtide binds to HIV-1 gp41, blocks conformational change required for fusion 32  Antiviral Drugs – Uncoating Amantadine blocks ion channels and inhibits uncoating of influenza virions Recall that M2 functions as an ion channel in the virion envelope to acidify the virion interior, required to free RNPs from matrix (M) so they can enter the nucleus This drug opened up new concepts in viral targets, targeting virus-encoded ion channels! Fig. 36.3 33  Antiviral Drugs – Nucleic Acid Synthesis Inhibitors Nucleoside analogues target genome replication General mechanism of action: incorporated into the growing DNA chain and prematurely terminate elongation or act as competitive inhibitors for nucleic acid synthesizing enzymes. NRTIs Used for herpesviruses Fig. 36.4 Used for HIV 34  Antiviral Drugs – Nucleic Acid Synthesis Inhibitors Example: Inhibition of herpes simplex virus (HSV) DNA polymerase by acyclovir triphosphate (pppACV in figure on next slide) → drugs must be specific to the viral polymerase to keep toxicity low Nucleoside analogues must first be activated by phosphorylation → mimics natural substrates (NTPs, dNTPs) Activation by viral kinases will cause the drug to be activated only in infected cells If viral polymerases are more sensitive to Fig. 36.5 drug, then concentration of drug can be kept low 35  Antiviral Drugs – Nucleic Acid Synthesis Inhibitors Mechanism of action for Acylovir → selectively phosphorylated by herpesvirus thymidine kinase: 1. Acyclovir triphosphate (pppACV) acts as a competitive inhibitor for the incorporation of deoxyguanosine triphosphate (pppdG) by DNA polymerase (green sphere) 2. ACV serves a substrate and is incorporated into growing DNA chain 3. DNA polymerase translocates to next position in template strand (dG), however there is no 3’OH on ACV to add the next deoxynucleoside triphosphate → chain termination Fig. 36.6 36  Antiviral Drugs – Nucleic Acid Synthesis Inhibitors Example: The HIV-1 NRTI azidothymidine (AZT) preferentially incorporates into proviral DNA, leading to chain termination AZT was the first drug approved for treatment of AIDS (1980) → clinical studies showed it could prolong life Nucleoside analogue with an altered sugar moiety (like ACV) The normal 3’OH replaced by N3 (azido) → causes chain termination in growing DNA Side effects: Higher toxicity than ACV because it’s phosphorylated by cellular kinases instead of viral kinases → drug activated in infected and uninfected cells 37  Antiviral Drugs – Nucleic Acid Synthesis Inhibitors Non-nucleoside RT inhibitors (NNRTIs) selectively target viral replication enzymes Nevirapine (NVP) is used for HIV-1 and binds close to the active site of viral RT and slows the rate of DNA polymerization Highly selective inhibitor (inactive against HIV-2) Virus rapidly derives resistance to NVP (mutations in RT) NVP has been used for prevention of mother-to-child transmission (PMTCT) of HIV → a single dose given to mother and child reduces the rate of transmission by 50% WHO endorses single-dose NVP prophylaxis in many developing world settings as a cost-effective means of PMTCT These drugs are generally used in combination therapies 38  Antiviral Drugs – Integrase Inhibitors Integrase inhibitors such as Raltegravir target HIV integrase by interacting with divalent metals within the active site → inhibits insertion of viral DNA into the host chromosome First approved inhibitor with this mechanism of action (FDA approval in 2007) Can be used as part of post-exposure prophylaxis (PEP) combinations 39  Antiviral Drugs – Protease Inhibitors Protease inhibitors can interfere with virus assembly and maturation Ritonavir: a successful protease inhibitor of HIV-1 → viral protease has an unusual substrate specificity (cleaves between Phe and Pro Aas, see below) that is not found in human proteases Peptidomimetic drug → mimics a peptide and iterative modifications to drug improved its pharmacokinetics (uptake, distribution, degradation, clearance) Designed based on natural substrate of HIV protease → a region in the Gag-Pol polyprotein Fig. 36.9, modified 40  Antiviral Drugs – Protease Inhibitors Designed based on natural substrate of HIV protease → a region in the Gag-Pol polyprotein Drug developed after crystal structure of HIV-1 protease was determined) Symmetrical inhibitors with the peptide bond replaced by hydroxyethylamine (compounds A-74702, A-74704) → mimic the original peptide and the catalytic intermediate that binds most tightly to the enzyme (protease) BUT cannot be cleaved! After many cycles of development → further increases in potency and improved pharmacokinetics led to final active Ritonavir Fig. 36.9 41  Antiviral Drugs – Neuraminidase Inhibitors Neuraminidase (NA) inhibitors inhibit the release and spread of influenza virus Recall that neuraminidase (NA) cleaves sialic acid from membrane glycoproteins → allows release of virus → inhibition = no release Zanamivir (Relenza) and Oseltamivir (Tamiflu) are effective in acute and/or severe disease (e.g. H5N1, H1N1) and are stockpiled for pandemics 42  Antiviral Drugs – HIV Targets Generally need combination therapy because virus latency and resistance development → need to hit multiple targets – Highly Active Antiretroviral Therapy (HAART) 43  Antiviral Drugs – SARS-CoV2 Targets Several drugs are under clinical trials 44 https://viralzone.expasy.org/9078b Antiviral Drugs – SARS-CoV2 Targets 45  Antiviral Drugs – Future Research on antiviral therapy is very active and rewarding Recent and expanding studies of virology are informing the development of new antivirals New antiviral targets → new drugs which can also be used to learn about virus biology in labs Many successes with influenza, HIV-1, and HSV → optimism that these successes can translate to other viral diseases! 46  Final Exam Final Exam: Saturday, December 7, 2 PM – 5 PM (double check online schedule in case there are any changes) Cumulative, with focus on Lecture 5-12. However, concepts from earlier lectures (1-4) will also be tested. Format same as midterm: MCQ, fill-in-the-blanks, SAQ (includes application questions) Room number and additional office hours for the final exam will be posted on Quercus. Thank you for a great semester! 47