NEW YORK (CNN/Money) - A coalition of 52 large employers are reportedly set to announce plans to pressure pharmacy-benefit managers to pass on more of the savings they win from drug manufacturers.

The Wall Street Journal reports that the coalition includes Caterpillar Inc. (Research), IBM (Research), Starbucks Corp. (Research) and Verizon Communications Corp. (Research) Together, the companies in the group spend $3.7 billion on drug purchases by 5 million employees and family members.

The companies plan to announce that they're endorsing a new purchasing model that would require pharmacy-benefit managers, or PBMs, to disclose and pass on to their clients their acquisition costs for retail and mail-order prescriptions, according to the newspaper.

Moreover, the group wants PBMs to pass along rarely disclosed rebates that they receive from pharmaceutical companies.

Executives with the employers told the newspaper they have certified three smaller PBMs that are willing to operate under these new rules. The companies will consider switching their business from their current PBMs unless they provide more disclosure, according to the report.

The country's biggest PBMs -- Medco Health Solutions Inc. (Research), Caremark Rx Inc. (Research) and Express Scripts Inc. (Research) -- buy drugs for millions of consumers, acting as intermediaries on behalf of their employer-customers to create large networks of participating pharmacies and drive down drug prices.

But the Journal reports a growing number of employers are wondering whether their PBM is always acting in the best interests of their clients.

According to the report, many employers say the complex and opaque financial arrangements PBMs have with drug manufacturers make it difficult to know whether they are always getting the best price, and that hidden rebates from the drug industry may lead PBMs to not always steer employees to the most cost-effective drug.

Medco declined to tell the newspaper which of the coalition's requirements it wouldn't meet. Express Scripts declined to comment, the newspaper reported, and Caremark didn't return phone calls.

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