Earlier this month, the second phase of clinical testing began for the COVID-19 vaccine that was jointly developed by Arcturus Therapeutics and Duke-NUS. The Phase Two study, which has been greenlighted by Singapore and US health authorities, aims to enrol 600 participants, with 100 to be recruited in Singapore.

The novel self-replicating messenger RNA (mRNA) vaccine, called LUNAR-COV19 or ARCT-021, has progressed to the next stage of development following positive Phase One/Two results. 

The study’s primary endpoints were the tolerability and safety of the vaccine. During a data call on 29 December (SGT), the company reported that only one serious adverse event had occurred in the placebo arm and no volunteers had withdrawn from the study.

Data on the participants’ immune responses, which was a secondary endpoint, indicated that the vaccine elicited a robust immune response. ARCT-021 triggered the development of both antibodies as well as T cells against SARS-coronavirus-2 (SARS-CoV-2), the cause of COVID-19.

While neutralising antibodies have been widely assumed to prevent viral infection, in preclinical challenge studies, in which animal models were vaccinated with ARCT-021 and then infected with SARS-CoV-2, ARTC-021 showed complete protection from infection only when CD8+ T cells, sometimes referred to as killer T cells, were present. 

“We found that even with low levels of antibody, the presence of virus-specific CD8+ T cells protected against infection,” said Ooi Eng Eong, a professor with Duke-NUS’ Emerging Infectious Diseases Programme, during the conference call. 

“One reason why we are seeing such good cellular immunity may be because the self-replicating vaccine presents the antigen over a longer period of time,” said Ooi. 

During the data call, Arcturus’ President and CEO Joseph Payne told analysts that “the science underlying ARCT-021 may provide meaningful advantages compared to other COVID-19 vaccine approaches”.

These advantages could be significant as they include the possibility of a single low-dose regimen as well as the option to be delivered in a powdered form that would do away with the need for a cold chain when transporting the vaccine.

The primary aim for the Phase Two study is to select the most promising dose to advance into Phase Three clinical trials. To do this as early as possible, Arcturus plans to conduct two interim analyses during the Phase Two study. For Phase Three, which the company targets to start recruitment for in the second half of 2021, the aim is to recruit 15,000 subjects.