* Humira contributes the bulk of AbbVie's sales
* AbbVie beats revenue, profit estimates for second quarter
* Stock down 5 percent in afternoon trade (Adds details from conference call, updates share movement)
By Tamara Mathias and Akanksha Rana
July 27 (Reuters) - AbbVie Inc's blockbuster arthritis drug Humira barely beat Wall Street sales forecasts, raising concerns about its future viability as a cash cow and sending the drugmaker's shares down 5 percent on Friday.
Humira is the world's best-selling prescription medicine and has long buoyed AbbVie's business, accounting for some two-thirds of overall sales. But as cheaper biosimilar versions come closer to entering the market, North Chicago, Illinois-based AbbVie could lose share and be forced to slash prices to remain competitive, analysts have said.
AbbVie executives took a barrage of questions from Wall Street analysts on Friday about future strategy in the backdrop of rising competition to Humira as well as looming U.S. policy changes aimed at tackling soaring healthcare costs.
AbbVie CEO Richard Gonzalez reiterated the company's current strategy of signing agreements with rival drugmakers to delay the launch of Humira copies, but investors seemed unconvinced.
AbbVie's stock dipped despite the company's stronger-than-expected profit for the second quarter of 2018 and a slightly higher annual earnings target.
"There's a lot of skepticism, concerns about rebates and biosimilars. Some of that is quite reasonable," Gonzalez told analysts on a conference call. "(But) every year since I can remember, there has been fretting about Humira."
Humira made $5.19 billion in second-quarter sales. Six analysts polled by Thomson Reuters I/B/E/S had on average expected $5.18 billion.
Concerns around Humira's future will only increase through 2018 despite AbbVie's "solid" overall results, Credit Suisse analyst Vamil Divan said in a report.
To rely less on Humira, AbbVie has tried to grow sales from other medicines including cancer treatment Imbruvica, which it acquired as part of a $21 billion deal in 2015. The drug raked in quarterly sales of $850 million and topped analysts' estimates.
AbbVie does not have any multi-billion dollar acquisitions on the horizon, Gonzalez said, in response to a question from an analyst.
Still, the company would consider buying smaller assets, he said. "It's not my objective to telegraph to all of our competitors what our defense strategies are. But what I can tell you is, thus far, it is playing out exactly as we had planned."
AbbVie's second-quarter revenue grew to $8.28 billion and beat analysts' estimates of $8.21 billion. Excluding one-time items, AbbVie earned $2 per share, edging past expectations of $1.97.
AbbVie now expects 2018 adjusted earnings of between $7.76 and $7.86 per share, compared with an earlier expectation of between $7.66 and $7.76. (Reporting by Tamara Mathias and Akanksha Rana in Bengaluru; Editing by Sai Sachin Ravikumar)