Clinigen said the deal would accelerate its global ambitions in the ethical supply of unlicensed medicines - drugs that are either in clinical trials or are licensed in other markets - to doctors who want to use them for their patients.
Chief Executive Shaun Chilton said it would also increase its ability to convert unlicensed drugs into licensed products.
"(Quantum) introduces an unlicensed medicine in the initial stages to cope with an unmet need and it converts it into a licensed medicine," he said in an interview.
"That unlicensed-to-licensed piece acts as a bridge between two of our business operations: our unlicensed medicines division and our commercial medicines division."
Quantum shareholders will receive 37 pence in cash and 0.0405 new Clinigen shares for each share, valuing each Quantum share at 82 pence, Clinigen said.
Quantum, which installed a new management team in October 2016 after a period of underperformance, reported revenue from continuing operations of 36.2 million pounds for the six months to end-July and adjusted core earnings of 5.7 million pounds.
Shares in Quantum rose 17.5 percent to 78 pence in early deals. Clinigen was trading down 1.4 percent.
(Reporting by Paul Sandle; editing by Jason Neely)