By Frances Bishopp

Hybridon Inc. has raised $50 million through the sale of 9-percent convertible subordinated notes due in 2004, at which time the notes will be convertible into shares of common stock at a price of $7.0125 each.

The closing is expected to occur April 7, 1997.

Hybridon sold the stock at a price of $6.125 per share, at a 14.5-percent premium for the company. Hybridon's stock (NASDAQ:HYBN) closed Thursday at $6.250, down $0.125.

According to a company spokesman, the notes were sold to qualified buyers which were institutions managing more than $100 million. The approximately 12 buyers were all from the U.S. In 2004, the bond holders will have the option of converting the debt into equity or continuing to hold the paper, which is tradable.

In January 1996, Hybridon raised $50 million, half of which came from European investors, in an initial public offering (IPO) marked as one of the largest market debuts of the previous several years. The company sold 5 million shares for $10 each.

After the IPO, the company had approximately 23.7 million shares outstanding.

Prior to the IPO, Hybridon, of Cambridge, Mass., had raised $108 million privately in a five-year period.

Hybridon discovers and develops genetic medicines based on antisense technology. Antisense involves the use of synthetic segments of nucleic acid, or oligonucleotides, constructed through rational drug design to modulate protein expression by interacting at the genetic level with target messenger RNA.

Hybridon's lead product, GEM 91, for the treatment of HIV-1 infection and AIDS, is in a confirmatory Phase II clinical trial in advanced AIDS patients. This trial is designed to confirm the preliminary findings from the company's Ib/II clinical trial of GEM 91 in the U.S., in which a significant decrease was observed in the quantities of cell-associated HIV-1 in circulating blood cells of patients with advanced HIV disease.

Hybridon also is conducting clinical trials of GEM 132, an advanced chemistry antisense compound for the treatment of cytomegalovirus (CMV). The first trial involves the treatment of CMV retinitis in AIDS patients by intravitreal injection in the eye; the second trial involves the treatment of systemic CMV by intravenous administration.

Hybridon will begin clinical trials of three additional products in the second half of 1997: GEM 231, an antisense compound being developed to inhibit the production of protein kinase A; GEM 92, an antisense compound being developed for treatment of HIV-1 infections and AIDS by oral administration; and GEM 220, an antisense compound being developed to target vascular endothelial growth factor for the treatment of various cancers.

In January 1996, Hybridon entered into a long-term drug development alliance with G.D. Searle & Co., of Skokie, Ill., with a potential worth to Hybridon of more than $200 million. The agreement covered the use of Hybridon's antisense technology to develop compounds for eight molecular targets identified by Searle as regulators of immune system responses.

Hybridon has other corporate collaborations with Roche Holdings Ltd., of Basel, Switzerland, Medtronic Inc., of Minneapolis, and Pharmacia Biotech AB, of Uppsala, Sweden, a subsidiary of Pharmacia & Upjohn Co., of Kalamazoo, Mich.

Hybridon and Roche are developing compounds for hepatitis B and C viruses and human papilloma virus. With Medronic, a medical device company, Hybridon signed a five-year deal in 1994 involving delivery of antisense compounds to the brain using Medtronic's implantable drug infusion. The companies are targeting Alzheimer's disease.

Hybridon's cash on hand, as of Dec. 31, 1996, was $16 million. *