But when he began a 10-day course of an antibiotic called roxithromycin for a chest infection, doctors were surprised to see his cancer-indicating paraprotein levels halve – and his haemoglobin levels increase.
Research haematologist Professor Ian Morison said he and his colleagues came across the response to the drug "completely by chance".
The improvement in his condition continued six months after.
"In untreated patients with myeloma the paraprotein levels are usually very stable, and the size of his reduction was extraordinary," Morison said.
"The persistent improvement in his haemoglobin strongly suggests that the amount of myeloma in his marrow has reduced."
The research team's interest in the response was supported by previous results from a related antibiotic, clarithromycin.
"Several years ago some remarkable responses to clarithromycin were observed, but subsequently others found modest responses and usually in combination with other treatments," Morison said.
"But the pharmacology of roxithromycin is much more favourable than clarithromycin so there are theoretical grounds for believing that it might be superior."
The findings were all the more exciting given how challenging myeloma is to deal with.
Plasma or myeloma cells are not very different that our normal plasma cells that make the antibodies of our immune systems.
But they grow slow - and cancers that are easier to treat with chemotherapy, such as acute leukaemia, tend to have cells that are faster growing.
"Chemotherapy kills many fast growing cells - including normal marrow cells, gut cells, hair cells - but these normal cells can recover," Morison explained.
"In the absence of differential cell growth, chemotherapy needs to be targeted to some specific feature of that cancer.
"The genetic abnormalities within myeloma cases are very heterogeneous, so that is no single cellular pathway that we can target for myeloma therapy."
But he added progress in treatment in myeloma had been "quite remarkable" in recent years.
New Zealand had reasonably early access, for instance, to bortezomib, a drug that had made a big difference to prognosis.
In the case of roxithromycin, Morison hoped its reported benefits might alert his team to similar agents from around the world.
"Many haematologists around New Zealand are aware of this case and are looking for similar responses."
Another haematologist from Japan had contacted the researchers to say he would try using the drug as well.
"If we obtain more anecdotal evidence of efficacy we will consider a formal trial of suitable patients."
Morison said roxithromycin was an easily affordable drug – a typical 10-day course cost just $3 – which meant drug companies would be unlikely interested in researching its activity in myeloma.
However, it wasn't used in many countries, which limited the opportunities for other scientists overseas to observe similar responses.
But the team was nonetheless keen to answer some of the most tantalising questions thrown up by the study.
"All of our excitement for the potential of roxithromycin comes from a single patient.
"Was there something else going on that we didn't know about? Is he the one-in-100 who responds to roxithromycin because of the genetic makeup of his myeloma cells?
"We need to see more cases with similar responses before we promote this as a treatment option."
