RSV vaccine: The primary goal is to identify, in phase 1 pediatric clinical trials, a live-attenuated strain of RSV suitable for further development as a pediatric RSV vaccine, to be given by intranasal administration. We previously showed that deletion of the ORF encoding the small (90 amino acids) viral M2-2 protein results in down-regulated viral RNA replication (causing viral attenuation) and up-regulated viral gene transcription and antigen synthesis, raising the possibility of increased immunogenicity. We recently evaluated a prototype virus bearing this mutation, called RSV MEDI/delM2-2, in a phase 1 clinical trial with clinical collaborators at Johns Hopkins University School of Public Health (JHU; NCT01459198). In seronegative children, compared to our previous lead RSV vaccine candidate rA2cp248/404/1030delSH, the delM2-2- virus was significantly more restricted for shedding in nasal washes but induced significantly higher titers of RSV neutralizing serum antibodies. Thus, it indeed appeared to have increased immunogenicity per infectious unit. Surveillance during the subsequent RSV season provided presumptive evidence of protection and strong anamnestic antibody responses. In the present review period, we completed a phase 1 clinical trial of a second delM2-2-based candidate, called RSV LID/M2-2. This was performed in 29 seronegative children 6-24 months of age (20 vaccine recipients and 9 placebo controls; NCT02040831 and NCT02237209). This study was done in collaboration with JHU plus the International Maternal Pediatric Adolescent AIDS Clinical Trials (IMPAACT) network. Unexpectedly, this virus was substantially less restricted than MEDI/delM2-2; the basis for this is presently being investigated. This virus also was more immunogenic. There was no evidence of increased respiratory tract illness in vaccine versus placebo recipients except that one vaccinee had brief, mild LRT illness. This was concurrent with the shedding of vaccine virus as well as enterovirus/rhinovirus. Therefore, causality was unclear. Surveillance during the following RSV season showed that 8 vaccinees had strong boosts in RSV-neutralizing serum antibody titers but without medically-attended RSV disease, indicating strong anamnestic responses to wt RSV without illness. These results confirmed that the delM2-2 mutation is associated with increased immunogenicity, apparent protection, and very strong amnestic responses. RSV LID/delM2-2 may be insufficiently attenuated. It is possible that a delM2-2 mutant with replication properties intermediate between the MEDI and LID viruses would be optimal. To this end, we presently are evaluating 2 additional delM2-2 viruses in phase 1 pediatric clinical trials, namely RSV D46/cp/delM2-2 (NCT02601612) and LID/delM2-2/1030s (NCT02794870). The rA2cp248/404/1030delSH virus mentioned above, which was a previous lead candidate, contains a series of point mutations and deletion of the SH gene, and is highly temperature-sensitive, but exhibits genetic instability involving at least 2 attenuating point mutations in the L protein called 248 and 1030. We previously developed new versions of these 2 mutations that were modified for increased genetic stability. A version of the rA2cp248/404/1030delSH virus was created that incorporated these stabilized mutations, resulting in a virus called RSV cps2. This virus was evaluated in a phase 1 pediatric clinical trial in seronegative children 6-24 months of age (34 vaccinees and 17 placebo recipients; NCT01852266 and NCT01968083), in collaboration with JHU and IMPAACT. Without going into detail, this showed that this virus indeed is less immunogenic than the delM2-2-based viruses noted above (which had been evaluated approximately in parallel), and thus this particular backbone will not be further developed at this time. However, sequence analysis of shed vaccine virus from vaccinees indicated an absence of de-attenuation at the attenuating mutation 248 or 1030, indicating that substantial stabilization of these mutations indeed had been achieved. These stabilized attenuating mutations presently are being used in other vaccine candidates (such as LID/delM2-2/1030s noted above). We also are evaluating another RSV vaccine candidate called RSV delNS2del1313. This virus contains the deletion of the nonstructural protein 2 (NS2) gene, whose encoded protein antagonizes host responses to viral infection, notably the type I interferon (IFN) response. It also contains deletion of codon 1313 in the polymerase L protein. This virus is being evaluated in a phase 1 pediatric clinical trial (NCT01893554) presently in progress. Thus, we have several promising RSV platforms in phase 1 pediatric clinical trials. Our goal is to perform a number of small phase 1 studies in 2016, 2017, and possibly 2018 to identify a lead candidate and a back-up candidate to bring forward to larger studies. Wild type (wt) RSV: We prepared a clinical trial lot of wt RSV strain A2 produced from cDNA. This provides a virus with a well-defined passage history and reduced possibility of adventitious agents. In collaboration with the Medical Virology Section, this virus presently is being evaluated in an in-patient setting for infectivity, replication, pathogenesis, and immunogenicity in healthy adult volunteers in a dose-escalation study (NCT02484417). This will provide an infection model that can be used to evaluate anti-RSV drug candidates and adult RSV vaccine candidates, and to study viral pathogenesis and the host response. HPIV1 vaccine: We previously developed an HPIV1 vaccine candidate called rHPIV1-C(R84G/del170)HN(T553A)L(Y942A) that includes mutations that were engineered for genetic stability. A phase 1 trial in seronegative children 6 to 59 months of age (NCT00641017) showed that this virus is over-attenuated. Over-attenuation can be corrected by reverse genetics. HPIV2 vaccine: We previously developed an HPIV2 vaccine candidate called rHPIV2-V94(15C)/948L/1724 that includes stabilized mutations. This virus presently is being evaluated in a phase 1 trial in seropositive children 15 to 59 months of age, which will be followed by evaluation in seronegative children 6 to 59 months of age (NCT01139437). HMPV vaccine: We previously developed an HMPV vaccine virus called rHMPV-Pa in which the HMPV P gene was replaced by that of avian MPV, thus conferring a host range attenuation phenotype. A phase 1 trial in seronegative children 6-59 months of age (NCT01255410) showed that this virus is over-attenuated. This provides a benchmark for pre-clinical comparison with other existing vaccine candidates to identify ones that are less attenuated. These were the first live-attenuated HPIV1, 2, and HMPV vaccines to be evaluated in humans.
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