Marape said there was nothing "illegal or improper" about the government's engagement with Niugini BioMed, a newly founded company which has said it was "highly confident" it had discovered a new treatment for Covid-19 after "scanning and analysing" 30,000 drugs from around the world.
The company is seeking K10.2 million for the "procurement of drugs and equipment for treatment for Covid-19".
Frank Me'alin, a Niugini BioMed board director and former parliamentary worker, said the treatment involved using existing medication already available in pharmacies.
"They've gone through the rigorous process already, they are out in the market."
Marape has said the process to engage Niugini BioMed "to find a cure for Covid-19" was not yet complete.
But Health Minister Puka Temu said the submission was approved, although he admitted he wasn't present when the decision was made.
"I believe that PNG hopefully with this initiative will add to the knowledge of Covid-19, which is lacking in the world today," Temu said.
Leaked documents from a cabinet submission outlined how a team of scientists who are part of the Niugini BioMed company — including several from the University of Papua New Guinea — had presented its findings in August to Marape, other government officials and the World Health Organisation's country director.
Dr Tauhid Islam, PNG WHO's officer in charge, said the organisation was not aware of the planned research program.
The cabinet submission recommended that Niugini BioMed work with PNG's Health Department, the PNG Medical Research Institute, as well as provincial health authorities and the national AIDS Council to "immediately run clinical trials".
But many of those bodies said they were also unaware of the cabinet submission, or any plans for their involvement.
William Pomat, director of the PNG Institute of Medical Research, said his organisation was not contacted for "advice and recommendations" by Niugini BioMed.
"I have not heard or been involved with any discussions with [this] group. I am concerned that the Institute of Medical Research has been overlooked in trying to get this through," he said.
Niugini BioMed was listed on PNG's business registry in mid-August, and Dr Peter Siba, former director of the institute and PNG's longest-serving virologist, believed the company lacked experience.
"I don't see any expertise, any credible people in there that are knowledgeable about viruses," he said.
"Secondly, there are institutions in the country that have the expertise, the equipment, infrastructure in place to be able to properly test and carry out research on these so-called miracle drugs prior to introducing onto the market".
The cabinet submission also recommended "annual financial support" by the government of PNG be approved.
Niugini BioMed's offices are registered to the University of PNG's (UPNG) chemistry division.
Niugini BioMed's Me'alin said he had contacted the Marape after noticing work done by scientists at UPNG.
"When I realised how significant it was, I had to reach out to the Prime Minister and his government to at least give them a chance to present the finding," Me'alin said.
The company's eight directors, many of whom are listed on its medical research team, include several university staff, including head of chemistry Reddy Kuama and Belly Asong, a University of PNG chemistry lecturer.
Marape said he was optimistic the medical team could find a successful Covid-19 treatment and praised the "young and competent PNG scientists" who might be "onto something big".
Temu also said the Health Department would ensure the company follows due processes.
"It will go through our health research medical advisory council, making sure the structure is right," he said.
Marape also signalled that he was confident a successful Covid-19 treatment could be found within PNG's borders.
"We are a nation of huge biodiversity, in fact, with 5% to 7% of the world's biodiversity. Our ancestors lived with malaria, snake bites and all manner of tropical diseases," Marape said.
PNG has recorded 589 cases of coronavirus since the pandemic began, but there is concern the infection rate could be much higher because of low testing numbers.