Chugai Pharmaceutical Co. Ltd., a Japanese drug...

Chugai Pharmaceutical Co. Ltd., a Japanese drug maker, has begun its previously announced $6.25-a-share, $115-million tender offer for Gen-Probe, the San Diego maker of medical products.

Chugai has an existing research-and-development agreement with Gen-Probe under which it is providing $15.5 million in R&D; funds, according to a filing with the Securities and Exchange Commission. The objective of the projects is to develop, over a five-year period, diagnostic products for eight types of viruses, and, over three years, of diagnostic products for two types of cancer.

Chugai now holds a 29.2% stake in Gen-Probe under a conditional option. All the funds required for the acquisition will be provided by Sumitomo Bank or with a capital contribution from Chugai Holdings, the company’s parent.

Merrill Lynch, Pierce Fenner, & Smith is the deal manager.