The 2003 Medicare Modernization Act provided for private insurers to offer more benefits to seniors, including prescription drug coverage that went into effect in January.

Officials with Humana, a 44-year-old health benefits company based in Louisville, KY, wanted to get a foothold in the emerging market in hopes of being the top private health insurer handling prescription drug plans.

Humana has offered Medicare replacement insurance to seniors for the past 20 years, but only in a few states. With the drug plan, ...