99999...77777 (281-digits)

This number is a probable-prime (likely to be a prime but we have not (re-)proven it on this site).

                                                                           9
9999999999 9999999999 9999999999 9999999999 9999999999 9999999999 9999999999
9999999999 8888888888 8888888888 8888888888 8888888888 8888888888 8888888888
8888555555 5555555555 5555555551 3333333336 6666666666 6666666666 6666666666
6666644444 4444444444 4777777777 7777777777 7777777777 7777777777 7777777777

Starting with the "most distinct digits prime" you can obtain two automorphic primes using the rule "d digits of each digit d" twice in cascade: 985130647 -> 9999999998888888855555133366666644447777777 -> etc. [Rivera]