You can deduct the interest on any loan that is used for business as a business expense, whether from a private lender or a bank. The most important part is to keep your documentation that shows you can trace the interest payment back to the specific business purpose, and don't mix loans.
(For example, you can even deduct credit card interest if you use the credit card for business purposes, but as soon as you put personal expenses on the same card, the interest becomes mixed up and the IRS would not look favorably on continuing to deduct the interest as a business expense.)
