1.1. DNA Vaccine Historical Context
The seminal studies of the DNA vaccine field, published in the early 1990s, demonstrated that administration of plasmid DNA to mice resulted in protein expression of encoded transgenes [
1], induction of antibody responses [
2], and protection from pathogenic challenge [
3]. Following these and other nonclinical proof of concept studies, the first-in-human phase 1 clinical trials were conducted during the mid-to-late 1990s in HIV-1-infected and normal healthy volunteers with DNA vaccines encoding HIV-1 and malaria antigens, respectively [
4,
5,
6]. Since then, DNA vaccines against other infectious disease agents have completed phase 1 and in some cases phase 2 testing including: anthrax, CMV, dengue, Ebola, hepatitis B virus, hepatitis C virus, herpes simplex virus-2, human papillomaviruses, seasonal and pandemic influenza viruses, measles, severe acute respiratory syndrome, and West Nile virus.
The results of early clinical trials of DNA vaccine candidates indicated favorable safety and tolerability profiles and evidence of humoral and cell-mediated immune responses [
7,
8]. However, the perceived need by most investigators to improve the immunogenicity of DNA vaccines resulted in the development of strategies to enhance DNA vaccine performance in humans. These strategies fall into four non-mutually exclusive categories, including: (
1) plasmid design (e.g., codon optimization, promoter selection, inclusion of a genetic-adjuvant-encoding plasmid); (
2) formulations (e.g., polymer, cationic liposomes, PLGA microspheres); (
3) devices (e.g., Biojector
® 2000, particle-mediated epidermal delivery, electroporation); and (
4) heterologous prime-boost with a viral vector (e.g., NYVAC, MVA, adenovirus) or recombinant protein. Vical Incorporated (hereafter Vical) developed a CMV DNA vaccine candidate utilizing the first two of these strategies. Proof-of-concept testing of the vaccine in a phase 2 trial has been completed and the vaccine is expected to enter a pivotal phase 3 trial sponsored by Astellas Pharma, Incorporated (hereafter Astellas). A case study is presented here of the development pathway of this vaccine.
1.2. CMV Background, Unmet Medical Needs, and Previous Vaccine Development Efforts
Cytomegalovirus, a β-herpesvirus and the largest virus known to infect humans, initiates a predominantly asymptomatic infection in normal, healthy individuals that persists for life, mostly as a latent infection without evidence of viremia [
9,
10]. CMV infection rates are high worldwide; the CMV seroprevalence rate in the U.S. is ~60% in ≥6 year olds and increases with age [
11]. CMV can cause significant morbidity and mortality in certain high-risk situations including congenital infection of fetuses and infection of recipients of hematopoietic stem cell transplant (HCT) or solid organ transplant (SOT), where treatment-related immunosuppression provides opportunities for CMV replication following viral acquisition or reactivation from latency. Antiviral drugs are licensed for use in transplant recipients either prophylactically or preemptively (upon evidence of viremia as measured by PCR or antigenemia assay). Unfortunately, the use of first line ganciclovir-based drugs or second line foscarnet and cidofovir can result in substantial hematotoxicity and nephrotoxicity [
9]. While antiviral drugs have reduced the incidence of CMV end organ disease (EOD) in CMV
+ HCT recipients from 25% prior to their licensure to approximately 5% today [
9,
12], alternative measures for controlling CMV replication after transplantation without attendant drug toxicities are needed. Vaccines represent one such strategy.
Numerous phase 1 and phase 2 trials have been conducted with several CMV vaccine candidates but a vaccine has yet to be licensed for any indication [
10]. Vaccine candidates include live attenuated vaccines such as Towne strain and Towne/Toledo chimeric strains, subunit vaccines, most notably adjuvanted recombinant gB, and vectored vaccines including recombinant viral vectors using canarypox and alphavirus platforms, and plasmid DNA vaccines [
10]. The live attenuated CMV Towne vaccine, derived in 1975, provided proof of concept in SOT recipients for reduced CMV disease severity after transplantation; however, it has not provided efficacy against infection in transplant recipients or in women exposed to CMV in daycare settings, and further development of this vaccine appears to depend upon implementing strategies to improve its immunogenicity [
10]. Beginning in the 1990s, a recombinant gB vaccine produced in CHO cells was developed and tested in combination with an MF59 adjuvant in multiple clinical studies over the ensuing decades. Two recent randomized controlled phase 2 studies provided proof of concept for the importance of gB as a protective CMV antigen by demonstrating 50% efficacy in decreasing maternal CMV infection [
13] and a significant reduction in the duration of viremia as well as the duration of antiviral ganciclovir treatment in CMV seronegative (CMV
−) recipients of kidneys or livers from CMV
+ donors [
14]. Despite these results it is unclear whether gB adjuvanted by MF59 will continue further development for either indication. Other vaccine approaches such as viral vectored gB [
15] and/or pp65 [
16,
17] have completed phase 1 testing with evidence of safety and immunogenicity. It is conceivable that the recent recognition of the potential importance of creating vaccines that target the pentameric gH/gL/UL128/UL130/UL131 epithelial entry pathway may have contributed to redirected vaccine efforts by some companies, at least for prophylactic vaccines attempting to block both fibroblast (gB-mediated) and epithelial entry pathways [
10].