Which letter will replace the question mark (?) in the following series?

W8, S27, Q64, M125, ?

K216

According to the above alphabet position:

W8    ⇒ (23), 2³

S27   ⇒ (19), 3³

Q64   ⇒ (17), 4³

M125 ⇒ (13), 5³

?       ⇒ (11), 6³ = K216

(Prime number in descending order in 1st alphabet position and there is cube of consecutive numbers in 2nd alphabet position)