**2.3 Presatovir**

During viral entry, the F protein undergoes conformational changes to fuse with the host cell membrane [17]. Presatovir (GS-5806) is an orally bioavailable agent that inhibits these conformational changes, thereby blocking viral fusion [27]. It was found in a Phase 2a trial with healthy adults (Clinicaltrials.gov identifier

#NCT01756482) to reduce viral load and severity of disease. However, it also caused low neutrophil counts and increased levels of alanine aminotransferase [27]. Despite these adverse events and because of its promise as an efficacious antiviral agent, a Phase 2b, randomized, double-blind trial in RSV-infected hospitalized adults was completed in April 2017 (Clinicaltrials.gov identifier #NCT02135614). The primary outcome was the time-weighted average change in RSV load from baseline to Day 5. There appeared to be no significant differences between Presatovir and placebo (−0.77 vs. −0.89, respectively, *p* value = 0.46).

#### **3. Currently available and under development immuno-prophylaxis**

#### **3.1 RSV-IVIG**

RSV Immunoglobulin (RSV-IVIG, RespiGam) is a pooled hyperimmune polyclonal immunoglobulin preparation made from donors with high titers of anti-RSV antibodies. RSV-IVIG significantly reduced morbidity and mortality in high-risk infants [28]. It was initially licensed in 1996, but taken off the market in 2004, due to the need for long intravenous infusion sessions and supervision in a hospital setting, high volume doses resulting in fluid overload in already at-risk infants, and potential risk for blood-borne pathogens [29]. Furthermore, immunizations with live-attenuated viruses, such as the measles/mumps/rubella (MMR) vaccine, need to be postponed until 9 months after RSV-IVIG infusion.

ALX-0171 is an inhaled trivalent nanobody that targets the RSV F protein [30]. A Phase I/IIa in RSV-infected infants and toddlers was recently completed in February 2016 (Clinicaltrials.gov identifier #NCT02309320). A Phase II dose ranging study RSV-infected hospitalized infants was recently completed in May 2018. Results from both studies have not been published yet.

#### **3.2 Palivizumab and motavizumab**

Palivizumab (Synagis), developed by MedImmune (Gaithersburg, MD, USA) in 1998, is the only currently approved prophylaxis agent against RSV infection [31]. It has been shown to reduce severe RSV infections by 55% and reduce RSV hospitalizations by 50%. Palivizumab is a humanized monoclonal IgG1 antibody that recognizes the RSV F protein and is administered intramuscularly monthly, for a maximum of 5 months, during the RSV season. It has no significant adverse side effects and other required live-attenuated vaccines can still be administered. However, because of the high cost, it is selectively given to high-risk infants: preterm infants born at <29 weeks of gestation; infants with chronic lung disease (CLD) of prematurity defined as gestational age <32 weeks of gestation and requirement of supplemental oxygen for the first 28 days of life; hemodynamically significant congenital heart disease; and might be considered for neuromuscular disorders that impair the airway clearance [32, 33].

Motavizumab (MEDI-524, Numax), an affinity-matured derivative of palivizumab, was shown to be more efficient than palivizumab with higher virus neutralizing effects [34]. However, it failed to receive FDA approval due to lack of greater clinical efficacy compared to palivizumab and cutaneous hypersensitivity reactions in some treated infants [35].

#### **3.3 Suptavumab**

Suptavumab (REGN2222) completed a Phase III trial in July 2017 (Clinicaltrials. gov identifier #NCT02325791). It is a human monoclonal IgG1 antibody against

**65**

*RSV: Available Prophylactic Options and Vaccines in Clinical Trials*

RSV-F [36]. 1177 preterm infants for whom palivizumab was not recommended were randomly assigned to one of three groups: Group 1 received one dose of intramuscular suptavumab and one dose of placebo, Group 2 received two doses of suptavumab, and Group 3 received two doses of placebo. There were no significant differences between the three groups in terms of the primary outcome of preventing medically attended RSV infection up to Day 150 [36]. All further development

MEDI8897 is another recombinant human monoclonal antibody with a modified Fc region that extends its half-life. MEDI8897 is being developed as RSV prophylaxis for all infants. The phase I (Clinicaltrials.gov identifier #NCT02114268) of study recruited 136 healthy adults, who received either MEDI8897 or placebo intravenously or intramuscularly, a single dose of 300–3000 mg. The half-life of the antibody was 85–117 days across the groups [37]. The phase Ib/IIa of the study, recruited healthy preterm infants with a gestational age of 32–35 weeks. The antibody group received as single intramuscular dose of 10–50 mg MEDI8897. The half-life of the antibody was 62.5–72.9 days. The authors concluded that the antibody has a favorable safety profile and can be administered as single dose during RSV season [38]. A Phase IIb trial in preterm infants' ineligible for Synagis was completed in 2018 and there is a plan for the

To date, there is no vaccine against RSV. Developing a vaccine against RSV remains a challenge, as the proper balance is required in eliciting an immune response, while avoiding vaccine-enhanced disease. While many of the proteins within RSV are being manipulated in different vaccine strategies, RSV F comprises a highly conserved amino acid sequence called antigenic site II, between RSV-A and RSV-B antigenic subgroups, and has been considered an important antigen for an RSV vaccine.

Designing a vaccine against RSV requires careful considerations. Infants, the elderly, and pregnant women are the three targeted populations for RSV vaccine development [39]. Each of the three types of vaccines, live-attenuated, vector delivery, and protein based, have benefits and drawbacks that have to be considered when developing vaccine technology (**Table 1**). Live-attenuated vaccines contain extracted components of viral proteins and present antigens most similarly to the naturally occurring infection [40]. They stimulate both humoral and cell-mediated immune responses. Live-attenuated vaccines are employed against many viral diseases, like measles, rubella, polio, rotavirus, varicella, and yellow fever.

Taken from: Rezaee F, Linfield DT, Harford TJ, Piedimonte G. Ongoing developments in RSV prophylaxis: a clinician's analysis. *Curr Opin Virol*. 2017;**24**:70–78.) One major drawback of live attenuated vaccines is that they cannot be given to patients with compromised immunity including pregnant woman. Vector-delivery system vaccines utilize a non-pathogenic virus genome with inserted portions of RSV proteins. Similar to live-attenuated vaccines, these vaccines increase mucosal IgA and cellular immune responses, yet without the risk of insufficient attenuation [40]. Protein-based vaccines include whole-inactivated viruses, subunit antigens, and particle-based vaccines. Live-attenuated or vector vaccines hold the greatest promise for infants due to the risk of vaccine-enhanced RSV disease. Pregnant women and the elderly are not susceptible to vaccine-enhanced RSV disease, and therefore protein-based RSV vaccines are likely the most effective candidates [40].

Phase III trial in healthy full-term and late pre-term infants in 2019.

**4. RSV vaccines under development**

*DOI: http://dx.doi.org/10.5772/intechopen.84851*

of Suptavumab has been stopped.

**3.4 MEDI8897**

RSV-F [36]. 1177 preterm infants for whom palivizumab was not recommended were randomly assigned to one of three groups: Group 1 received one dose of intramuscular suptavumab and one dose of placebo, Group 2 received two doses of suptavumab, and Group 3 received two doses of placebo. There were no significant differences between the three groups in terms of the primary outcome of preventing medically attended RSV infection up to Day 150 [36]. All further development of Suptavumab has been stopped.

## **3.4 MEDI8897**

*The Burden of Respiratory Syncytial Virus Infection in the Young*

to be postponed until 9 months after RSV-IVIG infusion.

Results from both studies have not been published yet.

**3.2 Palivizumab and motavizumab**

in some treated infants [35].

**3.3 Suptavumab**

**3.1 RSV-IVIG**

#NCT01756482) to reduce viral load and severity of disease. However, it also caused low neutrophil counts and increased levels of alanine aminotransferase [27]. Despite these adverse events and because of its promise as an efficacious antiviral agent, a Phase 2b, randomized, double-blind trial in RSV-infected hospitalized adults was completed in April 2017 (Clinicaltrials.gov identifier #NCT02135614). The primary outcome was the time-weighted average change in RSV load from baseline to Day 5. There appeared to be no significant differences between Presatovir and placebo (−0.77 vs. −0.89, respectively, *p* value = 0.46).

**3. Currently available and under development immuno-prophylaxis**

RSV Immunoglobulin (RSV-IVIG, RespiGam) is a pooled hyperimmune polyclonal immunoglobulin preparation made from donors with high titers of anti-RSV antibodies. RSV-IVIG significantly reduced morbidity and mortality in high-risk infants [28]. It was initially licensed in 1996, but taken off the market in 2004, due to the need for long intravenous infusion sessions and supervision in a hospital setting, high volume doses resulting in fluid overload in already at-risk infants, and potential risk for blood-borne pathogens [29]. Furthermore, immunizations with live-attenuated viruses, such as the measles/mumps/rubella (MMR) vaccine, need

ALX-0171 is an inhaled trivalent nanobody that targets the RSV F protein [30].

Palivizumab (Synagis), developed by MedImmune (Gaithersburg, MD, USA) in 1998, is the only currently approved prophylaxis agent against RSV infection [31]. It has been shown to reduce severe RSV infections by 55% and reduce RSV hospitalizations by 50%. Palivizumab is a humanized monoclonal IgG1 antibody that recognizes the RSV F protein and is administered intramuscularly monthly, for a maximum of 5 months, during the RSV season. It has no significant adverse side effects and other required live-attenuated vaccines can still be administered. However, because of the high cost, it is selectively given to high-risk infants: preterm infants born at <29 weeks of gestation; infants with chronic lung disease (CLD) of prematurity defined as gestational age <32 weeks of gestation and requirement of supplemental oxygen for the first 28 days of life; hemodynamically significant congenital heart disease; and might be considered for neuromuscular disorders that impair the airway clearance [32, 33]. Motavizumab (MEDI-524, Numax), an affinity-matured derivative of palivizumab, was shown to be more efficient than palivizumab with higher virus neutralizing effects [34]. However, it failed to receive FDA approval due to lack of greater clinical efficacy compared to palivizumab and cutaneous hypersensitivity reactions

Suptavumab (REGN2222) completed a Phase III trial in July 2017 (Clinicaltrials. gov identifier #NCT02325791). It is a human monoclonal IgG1 antibody against

A Phase I/IIa in RSV-infected infants and toddlers was recently completed in February 2016 (Clinicaltrials.gov identifier #NCT02309320). A Phase II dose ranging study RSV-infected hospitalized infants was recently completed in May 2018.

**64**

MEDI8897 is another recombinant human monoclonal antibody with a modified Fc region that extends its half-life. MEDI8897 is being developed as RSV prophylaxis for all infants. The phase I (Clinicaltrials.gov identifier #NCT02114268) of study recruited 136 healthy adults, who received either MEDI8897 or placebo intravenously or intramuscularly, a single dose of 300–3000 mg. The half-life of the antibody was 85–117 days across the groups [37]. The phase Ib/IIa of the study, recruited healthy preterm infants with a gestational age of 32–35 weeks. The antibody group received as single intramuscular dose of 10–50 mg MEDI8897. The half-life of the antibody was 62.5–72.9 days. The authors concluded that the antibody has a favorable safety profile and can be administered as single dose during RSV season [38]. A Phase IIb trial in preterm infants' ineligible for Synagis was completed in 2018 and there is a plan for the Phase III trial in healthy full-term and late pre-term infants in 2019.

### **4. RSV vaccines under development**

To date, there is no vaccine against RSV. Developing a vaccine against RSV remains a challenge, as the proper balance is required in eliciting an immune response, while avoiding vaccine-enhanced disease. While many of the proteins within RSV are being manipulated in different vaccine strategies, RSV F comprises a highly conserved amino acid sequence called antigenic site II, between RSV-A and RSV-B antigenic subgroups, and has been considered an important antigen for an RSV vaccine.

Designing a vaccine against RSV requires careful considerations. Infants, the elderly, and pregnant women are the three targeted populations for RSV vaccine development [39]. Each of the three types of vaccines, live-attenuated, vector delivery, and protein based, have benefits and drawbacks that have to be considered when developing vaccine technology (**Table 1**). Live-attenuated vaccines contain extracted components of viral proteins and present antigens most similarly to the naturally occurring infection [40]. They stimulate both humoral and cell-mediated immune responses. Live-attenuated vaccines are employed against many viral diseases, like measles, rubella, polio, rotavirus, varicella, and yellow fever.

Taken from: Rezaee F, Linfield DT, Harford TJ, Piedimonte G. Ongoing developments in RSV prophylaxis: a clinician's analysis. *Curr Opin Virol*. 2017;**24**:70–78.)

One major drawback of live attenuated vaccines is that they cannot be given to patients with compromised immunity including pregnant woman. Vector-delivery system vaccines utilize a non-pathogenic virus genome with inserted portions of RSV proteins. Similar to live-attenuated vaccines, these vaccines increase mucosal IgA and cellular immune responses, yet without the risk of insufficient attenuation [40]. Protein-based vaccines include whole-inactivated viruses, subunit antigens, and particle-based vaccines. Live-attenuated or vector vaccines hold the greatest promise for infants due to the risk of vaccine-enhanced RSV disease. Pregnant women and the elderly are not susceptible to vaccine-enhanced RSV disease, and therefore protein-based RSV vaccines are likely the most effective candidates [40].


#### **Table 1.**

*Advantages and disadvantages of the main strategy categories for RSV vaccine development.*


#### **Table 2.**

*Current vaccine candidates undergoing clinical trials.*

Live-attenuated, vector, and protein-based vaccines each possess advantages and disadvantages. Because non-replicating vaccines may elicit enhanced disease in RSVnaïve infants during subsequent infection, replicating or vectored vaccines might be a better choice in this group [41, 42]. Additionally, active immunization for infants is challenging due to passive immunity received from the mother [43]. Because of these factors, different vaccines may be required for different target populations. Understanding these complexities is crucial in RSV vaccine advancement. We will now discuss in depth the different vaccine strategies and current clinical trials in each category. A list of the vaccine candidates is summarized in **Table 2**.

**67**

*RSV: Available Prophylactic Options and Vaccines in Clinical Trials*

children 6–24 months of age was completed in May 2018.

gov identifiers #NCT03422237 and #NCT03387137).

Aside from deleting the M2–2 gene, the NS2 gene is another potential "knockout" gene for a live-attenuated vaccine. The RSV NS2 gene is known to promote epithelial cell shedding and inhibit host IFN response [15]. ΔNS2/Δ1313/1314 L, a vaccine candidate with a deleted NS2 gene, is genetically stable and moderately temperature-sensitive [48]. Another candidate, RSV 6120/∆NS2/1030s, also has a deleted NS2 gene, in combination with the "1030s" missense mutation, which provides further restriction of replication. Both of these candidates are currently being assessed in both seropositive and seronegative children and infants (Clinicaltrials.

Strategies have also targeted the SH gene. The RSV SH gene has multiple func-

tions, including inhibiting cell apoptosis, inhibiting signals from TNF-α, and modifying membrane permeability [49]. One vaccine that has a complete deletion of the SH gene, rA2cp248/404/1030∆SH, demonstrated restricted antibody response in the subjects, as well as viral genotypic and phenotypic instability

The tragic results of the formalin-inactivated RSV vaccine in the 1960s spurred research in the development of live-attenuated vaccine candidates. The live virus has parts of the genome deleted and is passaged at gradually lower temperatures. Live-attenuated vaccines require a delicate balance: maintain sufficient viral genome RNA replication to illicit enough antibody response in RSV-naïve infants, yet with a low risk of deattenuation and no harmful effects [44]. Live-attenuated vaccines are, in theory, safe for RSV-naïve infants because it does not exacerbate future exposure to RSV. Furthermore, it may be administered intranasally, which can mimic a milder form of a natural infection, and lead to viral replication in the upper respiratory tract [40]. This will induce mucosal and humoral immunogenicity, despite the potential presence of maternal antibodies acquired transplacentally. Several live-attenuated RSV vaccine candidates have deletions of a large segment of the M2–2 gene. The M2–2 gene mediates the transition from transcription to RNA replication [14]. *In vitro* studies have shown that M2–2 gene deletion leads to decreased viral RNA replication, but increased F and G protein expression through transcription. This means that the virus is adequately attenuated, yet potentially could lead to augmentation of the neutralizing antibody response [14]. A Phase I study explored the safety of a LID ΔM2–2 vaccine, delivered intranasally to RSVseronegative infants (aged 6 to 24 months). This vaccine infected the subjects successfully, but the peak shedding titers were higher than wanted, and therefore the study was terminated [45, 46]. Further attenuation to the LID ΔM2–2 vaccine, to counter the high shedding titers, is currently under investigation. The LID ∆M2–21030s vaccine has a mutation conferring temperature sensitivity. A Phase I placebo-controlled study in RSV-seronegative infants aged 6 to 24 months (Clinicaltrials.gov identifier #NCT02794870) completed in July 2017, showed that roughly 60% of vaccine recipients and 27% of placebo recipients had solicited adverse events. Conclusions regarding the LID ∆M2–21030s vaccine have not yet been made. A Phase I LID cp ∆M2– 2 vaccine, which in comparison to the LID ∆M2–2 contains 5 amino acid substitutions, was terminated early in seronegative infants 6 to 24 months of age due to indication that the vaccine "did not meet the protocol criteria for a good vaccine candidate" (ClinicalTrials.gov identifier #NCT02890381). We believe that this is because only 6/11 patients in the vaccine arm of the trial were infected with the vaccine virus from Study Day 0–28, thereby suggesting that there was not a strong enough immune response against the vaccine. Another vaccine candidate is RSV D46/NS2/N/∆M2–2- HindIII that contains one point mutation in the NS2 and N proteins and a modified version of the M2–2 deletion [47]. A Phase I study in RSV-seronegative infants and

*DOI: http://dx.doi.org/10.5772/intechopen.84851*

**4.1 Live-attenuated vaccines**

### **4.1 Live-attenuated vaccines**

*The Burden of Respiratory Syncytial Virus Infection in the Young*

• Induces immunity

exposure

• Does not exacerbate future RSV

• Administered intranasally

• Induced potent cellular and humoral responses in a primate model and preclinical studies • Safer option than live attenuated vaccines in children with no risk of insufficient attenuation

• Maternal immunization could increase transplacental antibody transfer and provide immunity

*Advantages and disadvantages of the main strategy categories for RSV vaccine development.*

**Live-attenuated** (For young infants and children <24 months of age)

**Vector delivery system**

**Protein-based** (For pregnant women and elderly)

**Table 1.**

(For young infants and children <24 months of age)

**Vaccine type Current strategies Live-attenuated** M2–2 gene deletion

for infants

**Vector delivery system** Adenovirus vector

**Protein-based** Particle based vaccine

*Current vaccine candidates undergoing clinical trials.*

Live-attenuated, vector, and protein-based vaccines each possess advantages and disadvantages. Because non-replicating vaccines may elicit enhanced disease in RSVnaïve infants during subsequent infection, replicating or vectored vaccines might be a better choice in this group [41, 42]. Additionally, active immunization for infants is challenging due to passive immunity received from the mother [43]. Because of these factors, different vaccines may be required for different target populations. Understanding these complexities is crucial in RSV vaccine advancement. We will now discuss in depth the different vaccine strategies and current clinical trials in

LID ∆M2–21030s LID cp ∆M2–2

**Advantages Disadvantages**

• Need to obtain delicate balance between immunogenicity and adequate

• Prior exposure to the vector and immunological memory against common serotypes may reduce the immune

response and limit their use

• The potential oncogenicity and pathogenicity of some Adenovirus serotypes

• High risk of exacerbation for RSV-naïve

attenuation

infants

NS2 gene deletion ΔNS2/Δ1313/1314 L RSV 6120/∆NS2/1030s SH gene deletion MEDI–559 RSV cps2

GSK3389245A GSK3003891A VXA-RSV-f Ad26.RSV.preF PanAd3-RSV

MVA-RSV MVA-BN

RSV D46/NS2/N/∆M2–2-HindIII

Modified Vaccinia Ankara vector

F-protein nanoparticle Subunit vaccine MEDI-7510

each category. A list of the vaccine candidates is summarized in **Table 2**.

**66**

**Table 2.**

The tragic results of the formalin-inactivated RSV vaccine in the 1960s spurred research in the development of live-attenuated vaccine candidates. The live virus has parts of the genome deleted and is passaged at gradually lower temperatures. Live-attenuated vaccines require a delicate balance: maintain sufficient viral genome RNA replication to illicit enough antibody response in RSV-naïve infants, yet with a low risk of deattenuation and no harmful effects [44]. Live-attenuated vaccines are, in theory, safe for RSV-naïve infants because it does not exacerbate future exposure to RSV. Furthermore, it may be administered intranasally, which can mimic a milder form of a natural infection, and lead to viral replication in the upper respiratory tract [40]. This will induce mucosal and humoral immunogenicity, despite the potential presence of maternal antibodies acquired transplacentally.

Several live-attenuated RSV vaccine candidates have deletions of a large segment of the M2–2 gene. The M2–2 gene mediates the transition from transcription to RNA replication [14]. *In vitro* studies have shown that M2–2 gene deletion leads to decreased viral RNA replication, but increased F and G protein expression through transcription. This means that the virus is adequately attenuated, yet potentially could lead to augmentation of the neutralizing antibody response [14]. A Phase I study explored the safety of a LID ΔM2–2 vaccine, delivered intranasally to RSVseronegative infants (aged 6 to 24 months). This vaccine infected the subjects successfully, but the peak shedding titers were higher than wanted, and therefore the study was terminated [45, 46]. Further attenuation to the LID ΔM2–2 vaccine, to counter the high shedding titers, is currently under investigation. The LID ∆M2–21030s vaccine has a mutation conferring temperature sensitivity. A Phase I placebo-controlled study in RSV-seronegative infants aged 6 to 24 months (Clinicaltrials.gov identifier #NCT02794870) completed in July 2017, showed that roughly 60% of vaccine recipients and 27% of placebo recipients had solicited adverse events. Conclusions regarding the LID ∆M2–21030s vaccine have not yet been made. A Phase I LID cp ∆M2– 2 vaccine, which in comparison to the LID ∆M2–2 contains 5 amino acid substitutions, was terminated early in seronegative infants 6 to 24 months of age due to indication that the vaccine "did not meet the protocol criteria for a good vaccine candidate" (ClinicalTrials.gov identifier #NCT02890381). We believe that this is because only 6/11 patients in the vaccine arm of the trial were infected with the vaccine virus from Study Day 0–28, thereby suggesting that there was not a strong enough immune response against the vaccine. Another vaccine candidate is RSV D46/NS2/N/∆M2–2- HindIII that contains one point mutation in the NS2 and N proteins and a modified version of the M2–2 deletion [47]. A Phase I study in RSV-seronegative infants and children 6–24 months of age was completed in May 2018.

Aside from deleting the M2–2 gene, the NS2 gene is another potential "knockout" gene for a live-attenuated vaccine. The RSV NS2 gene is known to promote epithelial cell shedding and inhibit host IFN response [15]. ΔNS2/Δ1313/1314 L, a vaccine candidate with a deleted NS2 gene, is genetically stable and moderately temperature-sensitive [48]. Another candidate, RSV 6120/∆NS2/1030s, also has a deleted NS2 gene, in combination with the "1030s" missense mutation, which provides further restriction of replication. Both of these candidates are currently being assessed in both seropositive and seronegative children and infants (Clinicaltrials. gov identifiers #NCT03422237 and #NCT03387137).

Strategies have also targeted the SH gene. The RSV SH gene has multiple functions, including inhibiting cell apoptosis, inhibiting signals from TNF-α, and modifying membrane permeability [49]. One vaccine that has a complete deletion of the SH gene, rA2cp248/404/1030∆SH, demonstrated restricted antibody response in the subjects, as well as viral genotypic and phenotypic instability

primarily due to reversion of the 1030 mutation [42, 48]. MEDI-559 differs from rA2cp248/404/1030∆SH by silent nucleotide substitutions throughout the viral genome [42, 50]. A Phase I/IIa trial studying the safety and efficacy of MEDI-559, showed a higher incidence of medically attended LRTI in RSV seronegative infants 5 to <24 months of age and in infants 1 to <3 months of age regardless of baseline serostatus within 28 days, as compared to placebo [50]. RSV neutralizing antibodies were detected in 59% of MEDI-559 recipients, in comparison to 9% of placebo subjects. Interestingly, this microneutralization response was lower than the rA2*cp*248/404/1030ΔSH vaccine's response. Adverse events, most notably URTI, occurred in 67% MEDI-559 and 57% placebo recipients, which was not clinically significantly different. Further safety trials are warranted to determine the safety profile of MEDI-559 as there was increased incidence of medically attended LRTI.

In comparison to MEDI-559, RSVcps2 contains 5 nucleotide changes and 1 amino acid substitution. The level of attenuation of RSVcps2 and MEDI-559 was shown to be similar in a study in seronegative chimpanzees [48]. This study also showed that it was temperature-sensitive and phenotypically and genetically stable. A Phase I trial in RSV-seronegative, healthy 6–24 month old children demonstrated that RSVcps2 is safe and effective [51]. Furthermore, unlike MEDI-559, medically attended LRTI was not observed. There were no significant differences in the number of adverse events between the experimental and control groups. However, in comparison to rA2cp248/404/1030ΔSH, RSVcps2 had decreased levels of replication and immunogenicity. The study investigators believe that this is due to the 37 silent nucleotide differences between the two vaccine candidates [51]. An ideal candidate would therefore combine the genetic stability of RSVcps2 and the greater replication and immunogenicity of rA2cp248/404/1030ΔSH. Other ∆SH vaccine candidates include OE4 (RSV-A2-dNS1-dNS2-ΔSH-dGm-Gsnull-line19F) and DB1 (RSV-A2-dNS-ΔSH-BAF), which have both been found to be immunogenic in cotton rats [52, 53].

#### **4.2 Vector delivery systems**

Vaccine technology is currently utilizing adenovirus and non-pathogenic viral genomes that can act as immune potentiators of delivery systems. These vaccines contain inserted portions of RSV F, N, and M2–1 proteins [54]. Vector vaccines increase mucosal IgA and cellular immune responses similar to live-attenuated vaccine candidates, yet without the risk of insufficient attenuation [55]. Furthermore, adjuvants used with these vector vaccines could potentially enhance the immune response to the vaccine [56].

*GlaxoSmithKline's* ChAd155-RSV (GSK3389245A) and GSK3003891A are RSV vaccine candidates encoded by a chimpanzee-derived adenovector. A Phase II trial (Clinicaltrials.gov identifier #NCT02360475) evaluating GSK3003891A in healthy, non-pregnant women aged 18–45 years was recently completed. The study showed that GSK3003891A is both safe and immunogenic. However, a Phase II trial in healthy pregnant women and infants born to vaccinated mothers was canceled due to instability of the PreF antigen during manufacturing. A Phase I study investigating ChAd155- RSV in healthy adults aged 18 to 45 years was recently completed (Clinicaltrials.gov identifier #NCT02491463), and a Phase II study in RSV-seropositive infants aged 12–23 months is underway (Clinicaltrials.gov identifier #NCT02927873). Another adenoviral-vector based RSV vaccine candidate, VXA-RSV-f, expressing the F-protein and a dsRNA adjuvant, is recently completed a Phase I, placebo-controlled, doseranging study, using subjects aged 18–49 years. Results have not been released yet.

Adenoviruses of serotype 26 (Ad26) are engineered to comprise a nucleotide sequence encoding RSV F protein, which showed efficacy against RSV in mice and

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*RSV: Available Prophylactic Options and Vaccines in Clinical Trials*

again initiated a robust cellular immune response [61].

cotton rats [57]. Two Phase I, placebo-controlled studies assessed the administration of Ad26.RSV.FA2, given either once or twice, followed by Ad35.RSV.FA2, and vice versa, to adults aged 18–50 years. Ad26.RSV.FA2 was shown to be safe and well tolerated. There was also increased humoral and cellular immunity for 6 months. Ad26.RSV.preF differs by 5 amino acids and contains the pre-fusion conformation stabilized F protein, and showed increased immunogenicity in comparison to Ad26. RSV.FA2 in pre-clinical studies [58]. It is currently undergoing a Phase II clinical trials in adults aged 18–50 years and RSV-seropositive toddlers aged 12–24 months (Clinicaltrials.gov identifier #NCT03303625) and in healthy adults greater than age 60 (Clinicaltrials.gov identifier #NCT03339713). PanAd3-RSV, a vaccine based on the RSV viral proteins F, N and M2–1 encoded by Simian Adenovirus, completed a Phase I trial in subjects 18–75 years of age (ClinicalTrials.gov identifier #NCT01805921) in 2015, alongside a Modified Vaccinia Virus Ankara (MVA) non-replicating vector vaccine candidate. Both of these vector vaccines contain RSV

PanAd3-RSV and MVA-RSV were both safe and effective in cotton rats, mice, and calves [59] and immunogenic in a primate model [54]. Most adverse effects were mild to moderate, self-limiting at the site of injection and the study concluded that the vaccine was safe and immunogenic [60]. Despite the promising results, no current clinical trial is investigating these vaccine candidates. MVA-BN (modified Vaccinia Ankara—Bavarian Nordic) is another MVA-based vaccine undergoing investigation. In August 2018, Bavarian Nordic announced that in a Phase II trial in older adults the MVA-BN vaccine elicited broad antibody and T cell responses to both RSV subtypes that lasted 6 months. Furthermore, a booster shot 1 year later

Pregnant women and the elderly are not susceptible to vaccine-enhanced RSV disease like infants, and therefore RSV protein-based vaccines are most likely the most effective candidates. Protein-based vaccine candidates include wholeinactivated viruses, subunit antigens, and particle-based vaccines. Vaccinating a pregnant woman can provide passive immunity to the fetus, as RSV-neutralizing antibodies have been shown to pass from mother to fetus *in utero* [43]. The higher RSV neutralizing antibody in cord blood was associated with reduced risk of hospitalization and disease severity in RSV infection has been shown by several studies [62, 63]. A recent comprehensive study measured multiple serum neutralizing RSV of the infants presented with primary RSV infection and did not find a direct relationship between the disease severity and level of most of anti–respiratory syncytial virus (RSV) antibody titers. However, they found a significant inverse relationship between antibody titer to RSV F protein and disease severity [64]. This is particularly important as the post-fusion form of RSV F protein has been used in clinical trial [65]. Additionally, experimental studies have shown that RSV infection during pregnancy can alter the offspring's postnatal immunity and airway hyperresponsiveness [66]. Therefore, a protein-based vaccine not only provides immunization for the pregnant woman, but also for the fetus in utero and the offspring once baby is born. MEDI-7510 is a subunit RSV vaccine candidate that contains the post-fusion F glycoprotein, with or without a glucopyranosyl lipid A (a synthetic TLR-4 agonist) adjuvant [67]. A Phase IIb trial in adults aged 60 and older showed that the vaccine candidate was

immunogenic but did not protect the study population from RSV illness [68].

Novavax's RSV F-protein nanoparticle vaccine has been trialed in a few Phase I and II studies in healthy human adults and one study of subjects 24 to <72 months of age, and was found to be well-tolerated and immunogenic in all studies [69, 70].

*DOI: http://dx.doi.org/10.5772/intechopen.84851*

viral proteins F, N and M2–1.

**4.3 Protein-based vaccines**

#### *RSV: Available Prophylactic Options and Vaccines in Clinical Trials DOI: http://dx.doi.org/10.5772/intechopen.84851*

*The Burden of Respiratory Syncytial Virus Infection in the Young*

primarily due to reversion of the 1030 mutation [42, 48]. MEDI-559 differs from rA2cp248/404/1030∆SH by silent nucleotide substitutions throughout the viral genome [42, 50]. A Phase I/IIa trial studying the safety and efficacy of MEDI-559, showed a higher incidence of medically attended LRTI in RSV seronegative infants 5 to <24 months of age and in infants 1 to <3 months of age regardless of baseline serostatus within 28 days, as compared to placebo [50]. RSV neutralizing antibodies were detected in 59% of MEDI-559 recipients, in comparison to 9% of placebo subjects. Interestingly, this microneutralization response was lower than the rA2*cp*248/404/1030ΔSH vaccine's response. Adverse events, most notably URTI, occurred in 67% MEDI-559 and 57% placebo recipients, which was not clinically significantly different. Further safety trials are warranted to determine the safety profile of MEDI-559 as there was increased incidence of medically attended LRTI. In comparison to MEDI-559, RSVcps2 contains 5 nucleotide changes and 1 amino acid substitution. The level of attenuation of RSVcps2 and MEDI-559 was shown to be similar in a study in seronegative chimpanzees [48]. This study also showed that it was temperature-sensitive and phenotypically and genetically stable. A Phase I trial in RSV-seronegative, healthy 6–24 month old children demonstrated that RSVcps2 is safe and effective [51]. Furthermore, unlike MEDI-559, medically attended LRTI was not observed. There were no significant differences in the number of adverse events between the experimental and control groups. However, in comparison to rA2cp248/404/1030ΔSH, RSVcps2 had decreased levels of replication and immunogenicity. The study investigators believe that this is due to the 37 silent nucleotide differences between the two vaccine candidates [51]. An ideal candidate would therefore combine the genetic stability of RSVcps2 and the greater replication and immunogenicity of rA2cp248/404/1030ΔSH. Other ∆SH vaccine candidates include OE4 (RSV-A2-dNS1-dNS2-ΔSH-dGm-Gsnull-line19F) and DB1 (RSV-A2-dNS-ΔSH-BAF),

which have both been found to be immunogenic in cotton rats [52, 53].

Vaccine technology is currently utilizing adenovirus and non-pathogenic viral genomes that can act as immune potentiators of delivery systems. These vaccines contain inserted portions of RSV F, N, and M2–1 proteins [54]. Vector vaccines increase mucosal IgA and cellular immune responses similar to live-attenuated vaccine candidates, yet without the risk of insufficient attenuation [55]. Furthermore, adjuvants used with these vector vaccines could potentially enhance the immune

*GlaxoSmithKline's* ChAd155-RSV (GSK3389245A) and GSK3003891A are RSV vaccine candidates encoded by a chimpanzee-derived adenovector. A Phase II trial (Clinicaltrials.gov identifier #NCT02360475) evaluating GSK3003891A in healthy, non-pregnant women aged 18–45 years was recently completed. The study showed that GSK3003891A is both safe and immunogenic. However, a Phase II trial in healthy pregnant women and infants born to vaccinated mothers was canceled due to instability of the PreF antigen during manufacturing. A Phase I study investigating ChAd155- RSV in healthy adults aged 18 to 45 years was recently completed (Clinicaltrials.gov identifier #NCT02491463), and a Phase II study in RSV-seropositive infants aged 12–23 months is underway (Clinicaltrials.gov identifier #NCT02927873). Another adenoviral-vector based RSV vaccine candidate, VXA-RSV-f, expressing the F-protein and a dsRNA adjuvant, is recently completed a Phase I, placebo-controlled, doseranging study, using subjects aged 18–49 years. Results have not been released yet. Adenoviruses of serotype 26 (Ad26) are engineered to comprise a nucleotide sequence encoding RSV F protein, which showed efficacy against RSV in mice and

**4.2 Vector delivery systems**

response to the vaccine [56].

**68**

cotton rats [57]. Two Phase I, placebo-controlled studies assessed the administration of Ad26.RSV.FA2, given either once or twice, followed by Ad35.RSV.FA2, and vice versa, to adults aged 18–50 years. Ad26.RSV.FA2 was shown to be safe and well tolerated. There was also increased humoral and cellular immunity for 6 months. Ad26.RSV.preF differs by 5 amino acids and contains the pre-fusion conformation stabilized F protein, and showed increased immunogenicity in comparison to Ad26. RSV.FA2 in pre-clinical studies [58]. It is currently undergoing a Phase II clinical trials in adults aged 18–50 years and RSV-seropositive toddlers aged 12–24 months (Clinicaltrials.gov identifier #NCT03303625) and in healthy adults greater than age 60 (Clinicaltrials.gov identifier #NCT03339713). PanAd3-RSV, a vaccine based on the RSV viral proteins F, N and M2–1 encoded by Simian Adenovirus, completed a Phase I trial in subjects 18–75 years of age (ClinicalTrials.gov identifier #NCT01805921) in 2015, alongside a Modified Vaccinia Virus Ankara (MVA) non-replicating vector vaccine candidate. Both of these vector vaccines contain RSV viral proteins F, N and M2–1.

PanAd3-RSV and MVA-RSV were both safe and effective in cotton rats, mice, and calves [59] and immunogenic in a primate model [54]. Most adverse effects were mild to moderate, self-limiting at the site of injection and the study concluded that the vaccine was safe and immunogenic [60]. Despite the promising results, no current clinical trial is investigating these vaccine candidates. MVA-BN (modified Vaccinia Ankara—Bavarian Nordic) is another MVA-based vaccine undergoing investigation. In August 2018, Bavarian Nordic announced that in a Phase II trial in older adults the MVA-BN vaccine elicited broad antibody and T cell responses to both RSV subtypes that lasted 6 months. Furthermore, a booster shot 1 year later again initiated a robust cellular immune response [61].

#### **4.3 Protein-based vaccines**

Pregnant women and the elderly are not susceptible to vaccine-enhanced RSV disease like infants, and therefore RSV protein-based vaccines are most likely the most effective candidates. Protein-based vaccine candidates include wholeinactivated viruses, subunit antigens, and particle-based vaccines. Vaccinating a pregnant woman can provide passive immunity to the fetus, as RSV-neutralizing antibodies have been shown to pass from mother to fetus *in utero* [43]. The higher RSV neutralizing antibody in cord blood was associated with reduced risk of hospitalization and disease severity in RSV infection has been shown by several studies [62, 63]. A recent comprehensive study measured multiple serum neutralizing RSV of the infants presented with primary RSV infection and did not find a direct relationship between the disease severity and level of most of anti–respiratory syncytial virus (RSV) antibody titers. However, they found a significant inverse relationship between antibody titer to RSV F protein and disease severity [64]. This is particularly important as the post-fusion form of RSV F protein has been used in clinical trial [65]. Additionally, experimental studies have shown that RSV infection during pregnancy can alter the offspring's postnatal immunity and airway hyperresponsiveness [66]. Therefore, a protein-based vaccine not only provides immunization for the pregnant woman, but also for the fetus in utero and the offspring once baby is born.

MEDI-7510 is a subunit RSV vaccine candidate that contains the post-fusion F glycoprotein, with or without a glucopyranosyl lipid A (a synthetic TLR-4 agonist) adjuvant [67]. A Phase IIb trial in adults aged 60 and older showed that the vaccine candidate was immunogenic but did not protect the study population from RSV illness [68].

Novavax's RSV F-protein nanoparticle vaccine has been trialed in a few Phase I and II studies in healthy human adults and one study of subjects 24 to <72 months of age, and was found to be well-tolerated and immunogenic in all studies [69, 70]. This vaccine consists of nearly the full-length F glycoprotein. This nanoparticle vaccine prompted transplacental antibody transfer within a guinea pig model [71]. Furthermore, in a Phase II study in healthy women of child-bearing age, the vaccine was well tolerated. The peak of Anti-F IgG antibody was day 14 and persisted for 3 months, optimal for administration during the third trimester [72]. Recently, the immunogenicity, with an aluminum adjuvant, was evaluated in a Phase II trial (Clinicaltrials.gov identifier #NCT02247726) in healthy third-trimester pregnant women. In this study in pregnant women, the primary outcome measures were safety and immunogenicity of the vaccine, as well as its impact on the number of infants with medically-attended RSV LRTI and age of onset of the infection. No results have been posted for this study. However, a Phase III study investigation in the same study population is set to be completed in 2019, thereby suggesting that the Phase II trial met its goals.
