* Aspen denies any wrong doing, committed to constructive engagement
* Roche says will cooperate fully with authorities
* Roche suspected of over-charging breast cancer drugs
* Lung cancer medicines sold by Pfizer excessive says Commission (Updates with Roche and Aspen comment, details)
PRETORIA, June 13 (Reuters) - South Africa's competition watchdog has launched an investigation into three drug companies accused of over-charging for cancer medicines, the agency's chief said on Tuesday.
Tembinkosi Bonakele, head of the Competition Commission, said the agency would investigate Aspen Pharmacare, Africa's biggest generic drug maker, U.S. company Pfizer and Swiss-based Roche Holding.
"Here we have a suspicion. We think that the reason is excessive pricing by the participants in the market. We have to investigate and bring people to book," Bonakele told a news conference.
"The Competition Commission has identified the healthcare sector, and in particular, pharmaceuticals, as a priority sector for its enforcement efforts due to the likely negative impact that anti-competitive conduct in that sector would have on consumers in general and specifically the poor and vulnerable."
The Commission, which investigates cases before bringing them to the Competition Tribunal for adjudication, said it suspected the lung cancer treatment xalkori crizotinib sold by Pfizer had been excessively priced as has the breast cancer drugs Herceptin and Herclon sold by Roche.
It also said it would look into whether Aspen, a local company based in Durban, might have over-charged for Leukeran, Alkeran and Myleran cancer treatments in South Africa.
Roche said in an email it had not received a formal notification from the Commission when asked for comment.
"In case we receive a formal notification, we will be cooperating fully with the authorities, will provide all required information and will respond to the allegations," the company said.
Pfizer did not immediately respond to telephone requests for comment.
Aspen denied any wrongdoing, saying it had not increased its prices for medicines used to treat leukaemia beyond the margin approved by the South African health department.
Some medicines in South Africa, including those sold by Roche and Aspen, are considered too essential to let manufacturers set the prices.
"Aspen is committed to full and constructive engagement with the Competition Commission in this investigation," the company said in a statement.
Aspen is already under investigation by the European Commission over allegations that it is overcharging for five key cancer drugs. (Additional reporting by Paul Arnold in Zurich Writing by Tiisetso Motsoeneng; Editing by Mark Potter, Greg Mahlich)