Three-year-old Madeleine disappeared from a holiday apartment in Praia da Luz, Portugal on 3 May, 2007.
Her mother Kate described the approaching 10th anniversary as a "horrible marker of time, stolen time".
Asked if police were any closer to solving the case than they were six years ago, Mr Rowley said: "I know we have a significant line of inquiry which is worth pursuing, and because it's worth pursuing it could provide an answer, but until we've gone through it I won't know whether we are going to get there or not."
But he added that there was no "definitive evidence" as to whether Madeleine was alive or dead.
He declined to expand on the nature of the working theories or reveal whether any suspects were currently being considered but said the investigating team were still receiving evidence and new information from members of the public on a daily basis.
He said UK and Portuguese detectives were doing "a critical piece of work and we don't want to spoil it by putting titbits of information out publicly".
But he added that "there was no reason whatsoever" to suspect Madeleine's parents were involved in her disappearance.
"However she left that apartment, she's been abducted," he said. "This is a young girl who is missing."
He said: "We've got some thoughts on what we think the most likely explanations might be and we are pursuing those."
Four detectives are working on the case in the UK, scaled back from 30 in 2011, with more than £11m spent so far.
Scotland Yard started its review of the case in May 2011, after then-Prime Minister David Cameron responded to a plea from the McCanns.
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