Complex sentence: Theatre producer Frederick Brisson originally had optioned Chanel's life for his wife Rosalind Russell, but with acute arthritis making it difficult for her to function, another leading lady with star quality needed to be found.
Split into simpler sentences: Theatre producer Frederick Brisson originally had optioned Chanel's life for his wife Rosalind Russell, but Russell had developed acute arthritis, making it difficult for her to function. So another leading lady with star quality needed to be found.