The Senate Special Committee on Aging led by US Senators Susan Collins and Claire McCaskill has launched a formal investigation at four pharmaceutical companies for increasing their drug prices in recent months.

After the acquisition or merger of pharmaceutical companies, the firms have often increased prices of older or off-patent drugs.

The companies including Valeant Pharmaceuticals, Turing Pharmaceuticals, Retrophin, and Rodelis Therapeutics have been asked to submit documents and information on drugs that have seen recent and significant spikes in price.

The committee is seeking to understand the causes, impacts and potential solutions related to the issue.

Collins said: "The sudden, aggressive price hikes for a variety of drugs used widely for decades affect patients and healthcare providers and the overall cost of healthcare.

"These substantial increases have the potential to inflate the cost of healthcare for Americans, especially our seniors, by hundreds of millions of dollars each year.

"These substantial increases have the potential to inflate the cost of healthcare for Americans by hundreds of millions of dollars each year."

"Given the potential harm to patients across our country who rely on these drugs for critical care and treatment, the Senate Special Committee on Aging considers these massive price increases worthy of a serious, bipartisan investigation into the causes, impacts, and potential solutions."

In September, Turing agreed to cut the price of Daraprim amid increasing protests at plans to increase it by 5,000%.

The price of toxoplasmosis drug Daraprim (pyrimethamine), acquired from Impax Laboratories in August for $55m, was increased by Turing from $13.50 to $750.

Daraprim is the only FDA-approved drug to treat toxoplasmosis, when used in combination with sulfonamide and leucovorin.

Turing Pharmaceuticals CEO Martin Shkreli said: "We seek your cooperation with this investigation so that the committee may better understand drug pricing and related regulatory and public policy concerns.

"In particular, the committee wishes to learn more about Turing Pharmaceuticals’ recent acquisition of the rights to sell Daraprim, a drug used to treat and prevent infections, from Impax Laboratories and Turing’s subsequent decision to increase the price of Daraprim from $13.50 to $750 (per tablet)."

In October, Valeant received two subpoenas from federal prosecutors in the US seeking information relating to its drug pricing, distribution and patient support practices.

Currently, the committee is seeking information relating to the company’s cardiac care products Nitropress and Isuprel, as well as Cuprimine, which is indicated to treat Wilson’s disease.

And investigation will also be carried out on the US Food and Drug Administration’s role in the approval process for generic drugs, the agency’s distribution protocols, and its off-label regulatory regime if necessary.

US-based biopharmaceutical firm Retrophin has been asked by the senators to provide information on the pricing of Thiola, a drug designed to treat kidney disease.

Retrophin acquired the rights to the drug from Mission Pharmacal and subsequently increased the price to $30 from $1.50 per tablet.

Furthermore, Rodelis has been asked to provide information related to its acquisition of the rights to sell Seromycin (cycloserine), a drug used to treat tuberculosis.