Three months in to Medicare Part D, the stand-alone drug insurance market is heavily concentrated. Not surprisingly, United has the biggest share. But one member of the top tier--MemberHealth--is an unlikely heavyweight.

As Part D gets under way, it is increasingly clear that the new Medicare drug benefit poses serious threats to Medco, Caremark and Express Scripts.

Humana thrilled CMS-and shocked its competitors-by offering stand-alone drug insurance plans under Medicare for less than $10 per month. Pharmaceutical manufacturers should applaud. Not all Part D strategies are equal for pharma companies. Manufacturers should compare Humana's approach to other aggressive bids by UnitedHealth and WellCare.

Pharmaceutical manufacturers interested in maximizing their market share under the new Medicare drug benefit in 2006 should focus on four insurers likely to take the lion's share of enrollment. Dozens of plans are entering the market, but UnitedHealth Group, Wellpoint, Humana and Wellcare will start with a big edge in enrollment as of January 1. An analysis by The RPM Report explains why.