You can use np.intersect1d
:
import pandas as pd
import numpy as np
frame = pd.DataFrame({'a' : ['a,b,c', 'a,c,f', 'b,d,f','a,z,c']})
letters = ['a','c']
mask = frame.a.apply(lambda x: np.intersect1d(x.split(','), letters).size > 0)
print(frame[mask])
a
0 a,b,c
1 a,c,f
3 a,z,c