When a parenthetical phrase or sentence interrupts the middle of a sentence, do not capitalize the first letter inside the parentheses unless, of course, the word is a proper noun or proper adjective.

Should words inside parentheses be capitalized?

Capitalize the first word in parentheses if it is a proper noun or the beginning of a complete sentence. Verify all items.

What is parenthetical element?

A parenthetical element is information that is nonessential to the meaning of a sentence, such as an example, a clarification, or an aside.