Alexandria Real Estate Equities Inc. late Monday said that it plans to pay down debt by selling 6 million new common shares in a public offering.

After the markets closed, the Pasadena real estate investment trust said it plans to use net proceeds to reduce the outstanding balance on its unsecured senior line of credit. It has not yet priced the shares, but if they were sold at Monday’s closing price, the stock sale would raise nearly $454 million. The company now has 63.7 million shares outstanding.

Alexandria owns 173 properties, totaling about 16.7 million rentable square feet, which it leases to drug and medical device companies, academic institutions and U.S. government research agencies. Most of its holdings are biotech hubs such as San Diego, San Francisco and Boston.

Shares earlier closed up 23 cents, or less than 1 percent, to $75.63 on the New York Stock Exchange, and fell 2.5 percent in after-hours trading.