Pennekamp has been in Dubai since January, preparing for an ambitious comeback in the inaugural running of the $4m contest, the world's richest race, at Nad Al Sheba.
But Anthony Stroud, Sheikh Mohammed's racing manager, said: "The old splint problem has recurred and the extent of the injury is such that he will be unable to make the race. At this stage it is difficult to ascertain how serious it is and what his future may be."
The absence of Pennekamp reduces the field to 11, headed by the American champion, Cigar. Pentire and Needle Gun will represent Britain, with Halling, Larrocha, Tamayaz and Torrential running for the United Arab Emirates.
Lord Relic is the latest horse to drop out of next week's Grand National. The 10-year-old is to be given a rest after being pulled up in last week's Cheltenham Gold Cup. Scotland's only remaining National entry, Emerald Storm, goes on trial for the race at Kelso today. The nine-year-old is a 100-1 chance for Aintree.Reuse content