OT: Anti-Perspirant stains on cotton Polo shirts - how to clean

A bit OT but thought a few of the regulars here would have some good suggestions. I have a number of Polo shirts I use for work. They are in fine shape except for the Anti-Antiperspirant stains in the armpits. I have tried soaking in detergent and using Shout! gel to pretreat. Any other suggestions. I hate to throw them out at $35 a pop.
Sweating in Arizona is not a option for 3/4 of the year. (SMILE!!)
Bart D. Hull
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