Finally, a few weeks ago, the National Comprehensive Cancer Network, or NCCN, published updated GIST Clinical Practice Guidelines and added a recommendation for the use of QINLOCK 150 milligrams twice daily dosing, or BID, upon disease progression on QINLOCK 150 milligrams once-daily dosing, or QD. An exploratory analysis of the INVICTUS Phase III study of QINLOCK in fourth-line and fourth-line plus GIST published in The Oncologist in November of last year showed that patients who received QINLOCK dose escalation to 150 milligrams BID from 150 milligrams QD after disease progression experienced substantial additional clinical benefit as measured by further progression-free survival.
I’ll now turn the call over to Dan Martin, our Chief Commercial Officer, to provide an update on our commercial efforts. Dan?
Daniel C. Martin
Senior VP & Chief Commercial Officer
Thank you, Steve. In Q4, we continued to execute on our commercial goals for QINLOCK, including reinforcing its position as the clear standard of care in fourth-line GIST, continuing to expand our prescriber footprint and providing this important treatment to patients with advanced GIST in the U.S. and globally.
In Q4, we achieved $23.7 million in total net product revenue globally, including $21.5 million in the U.S. The core drivers of QINLOCK demand remained consistent in the U.S. including new patient acquisition, payer access and persistency. During the quarter, our launch-to-date prescriber base grew to nearly 600 physicians, increasing 13% versus Q3 with most new prescribers again coming from the community setting.
Product attribute ratings among QINLOCK users remained exceedingly high in Q4 among both academic and community treaters. During the quarter, as anticipated, the percentage of patients receiving free drug under our patient assistance program, or PAP, was at the high end of our 20% to 30% estimated range and the gross to net adjustment was in line with our projected annual average of 15%. Our PAP in gross to net guidance for 2022 remains the same as in prior years. Lastly, as is common across the industry in the U.S., we saw a modest increase in channel inventory during Q4 that may have a limited impact on ex-factory sales and product revenue in Q1.
Turning to Europe. Now that we have regulatory approval in fourth-line GIST, our team is focused on achieving reimbursement in markets where we can launch most quickly. In Germany, we are now actively promoting QINLOCK, with patients receiving their first shipments last month. In Switzerland, we are pursuing named patient sales. And in France, we are transitioning to a post-approval paid access program in the first half of 2022 before we receive formal pricing and reimbursement decisions. As we have communicated previously, we believe the number of patients diagnosed with GIST each year across the 5 largest European markets is equivalent to the number of patients in the United States. We look forward to building on our U.S. launch success as we now introduce QINLOCK in Europe.
Turning to vimseltinib. As Steve noted, we believe there is significant unmet need and the opportunity to offer meaningful clinical benefit to patients with TGCT. And based on our market research, engagement with KOLs and analysis of U.S. claims data, we believe there is a significant market opportunity for a new therapy with a compelling efficacy and safety profile. We estimate that each year in the U.S., 14,000 to 18,000 patients are diagnosed with TGCT, 2,000 to 2,400 will recur after their first surgery and 1,300 to 1,400 will initiate systemic therapy.
Based on these figures and other insights gleaned from the claims data, we estimate a total potential addressable market in the U.S. of approximately $850 million. And this estimate does not include the additional opportunity associated with a significant prevalent population living with TGCT nor does it include the opportunity in Europe where there are no approved therapies, and the incidents and prevalence are expected to be proportional to the U.S.
Today, the only FDA-approved therapy for TGCT is pexidartinib, which has a boxed warning, a REMS program and extensive liver monitoring requirements because of known hepatotoxicity risk. Given this challenging risk-benefit profile, it’s perhaps not surprising that claims that show that few patients who are treated with systemic therapy received pexidartinib. Instead, the majority of these patients receive off-label imatinib despite imatinib not