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Operator
Good morning, ladies and gentlemen, and welcome the Supernus Pharmaceuticals fourth-quarter and full-year 2015 earnings conference call. (Operator Instructions) As reminder, this conference call is being recorded.
I would now like to turn the conference over to Peter Vozzo from Westwicke Partners. You may begin.
Peter Vozzo - IR, Westwicke Partners
Thank you, Karen. Good morning, everyone, and thank you for joining us today for Supernus Pharmaceuticals' fourth-quarter and full-year 2015 financial results conference call. Results discussed today are for the quarter and year ended December 31, 2015. Yesterday after the close of the market, the Company issued a press release announcing these results.
On the call with me today are Supernus' Chief Executive Officer Jack Khattar and Chief Financial Officer Greg Patrick. Today's call is being made available via the investor relations section of the Company's website at ir.supernus.com. Following remarks by management, we will open the call to questions. We expect the duration of the call to be approximately 45 minutes.
During the course of this call, management may make certain forward-looking statements regarding future events and the Company's future performance. These forward-looking statements reflect Supernus' current perspective on existing trends and information, and can be identified by such words as expect, plan, will, may, anticipate, believe, should, intend, and other words of similar meaning.
Any such forward-looking statements are not guarantees of future performance and involve risks and uncertainties, including those noted in the risk factors section of our annual report on Form 10-K, the most recent of which we will file next week. Actual results may differ materially from those projected in these forward-looking statements.
For the benefit of those who may be listening to the replay, this call is being held and recorded on March 3, 2016, at approximately 9 AM Eastern Time. Since then, the Company may have made additional announcements related to the topics discussed. Please reference the Company's most recent press releases and current filings with the SEC. Supernus declines any obligation to update these forward-looking statements, except as required by applicable securities laws.
I will now turn the call over to Jack.
Jack Khattar - President and CEO
Thank you, Peter. Good morning, everyone, and thanks for taking the time to join us as we discuss our 2015 fourth-quarter and full-year results. 2015 was a milestone year for Supernus. We continued to generate strong prescription and product sales growth while making significant progress in advancing clinical development of our pipeline products, including the initiation of our Phase 3 trials for SPN-810 and our Phase 2b trial for SPN-812. Based on the strength of our business, for the first time, the Company was profitable from ongoing operations for the full year as well as in each quarter of 2015.
We also filed a supplemental new drug application during the year requesting a label expansion for Trokendi XR to include treatment of migraines, which was accepted for review by the FDA.
Let me now focus on some of the key fourth-quarter highlights. Prescription growth for our epilepsy portfolio continued at a strong pace in the quarter. Total prescriptions for Trokendi XR and Oxtellar XR combined in the fourth quarter as reported by IMS were 111,627, representing an increase of 61% over the fourth quarter of 2015 and 11% over the third quarter of 2015.
Trokendi XR prescriptions for the fourth quarter totaled 83,899, which is a 71% increase over the fourth quarter of last year and 12% increase over the third quarter of 2015. Oxtellar XR prescriptions for the fourth quarter totaled 27,728, representing an increase of 39% over the fourth quarter of last year and an 8% increase over the third quarter of 2015. In addition, total net product sales for the fourth quarter of 2015 were $42.6 million, an increase of 40% over the same quarter last year and an 11% increase over the third quarter of 2015.
Looking now at our performance for the full year of 2015, the total prescriptions for Trokendi XR and Oxtellar XR showed a robust growth of 91% over full-year 2014. And the total net product sales increased by 60% or 74% when adjusted for the change in revenue recognition of $7 million in 2014 that we discussed on our last two quarterly calls.
As I mentioned, the FDA accepted for review our supplemental new drug application requesting approval to expand the indication for Trokendi XR beyond the current indication for the treatment of epilepsy to include treatment in adults for prophylaxis of migraine headache. This potentially provides an important new treatment option for adult patients suffering from this condition and helps us maximize the Trokendi XR franchise. The FDA has set a target date in the second quarter of 2016 to complete its review. In 2016, we plan to invest in the launch of the migraine indication to maximize the potential of Trokendi XR.
Turning now to our pipeline, let me first discuss SPN-810. During the fourth quarter of 2015, as we had planned, we finalized the special protocol assessment with the FDA, conducted an investigative meeting with approximately 50 participating centers covering both Phase 3 trials, and begin site initiation visits. We expect to recruit substantially all patients into both Phase 3 trials for SPN-810 during 2016. Patient dosing will start in this quarter and the Phase 3 data is expected to be available by mid-2017.
Regarding SPN-812, during the fourth quarter of 2015, final results were received from a single-ascending dose and multiple-ascending dose study in adult healthy volunteers. The data showed that doses of up to 1,800 milligram per day of the extended-release formulation were better tolerated than doses of up to 400 milligrams a day of the immediate-release formulation that was used in our Phase 2a trial.
We are very excited about the emerging clinical profile of SPN-812. These recent data from our SAD and MAD study reinforce our belief that SPN-812 as a novel non-stimulant has the potential for being dosed at doses high enough to compare favorably with stimulant medications for efficacy while also showing a favorable tolerability and safety profile. Patient dosing in the SPN-812 Phase 2b trial has started and data is expected to be available by early 2017.
Regarding our intellectual property for Oxtellar XR, we are pleased that a federal district court found that three patents covering Oxtellar XR are valid and that Octavius infringed two of them. In addition to these three patents that were found valid by the federal court decision, Oxtellar XR is further protected by two additional patents that are also listed in the Orange Book and that have an expiration date that is no earlier than 2027.
We remain confident in the strength of our intellectual property and continue to vigorously defend the patent protection of our innovative products that they deserve. Oxtellar XR has five patents and Trokendi XR has six patents that are listed in the Orange Book. All these patents provide patent protection that expires no earlier than 2027.
Finally, we continue to actively look for partnership and corporate development opportunities that strategically fit with our vision in building Supernus as a premier specialty pharma company.
With that, I will now turn it over to Greg to walk you through details on the financial results.
Greg Patrick - CFO
Thanks, Jack. And good morning, everyone. As I review our financial results, I would like to remind our listeners to refer to the fourth-quarter and full-year 2015 earnings press release issued yesterday after the market closed. We expect to file our report on Form 10-K for the year ended December 31, 2015, next week.
Net product sales of Trokendi XR for the fourth quarter of 2015 were $33.3 million, which is 11.4% higher than $29.9 million in the third quarter of 2015 and 45.5% higher than $22.9 million in the fourth quarter of 2014. Net product sales of Oxtellar XR in the fourth quarter of 2015 were $9.3 million, a 6.9% increase over $8.7 million in the third quarter of 2015 and a 22.4% increase over $7.6 million in the fourth quarter of 2014.
As Jack mentioned, total net product sales for full-year 2015 increased by 60% or 74% when adjusted for the change in revenue recognition of $7 million in 2014 related to Trokendi XR. We discussed this on our last two quarterly calls.
Net product sales for Trokendi XR for full-year 2015 when adjusted for this change in revenue recognition increased by 90% over full-year 2014, while net product sales for Oxtellar XR grew by 34% over the same period. Product gross margin for the quarter was 93.4% and for the year was 94.1% compared to 92.5% and 93.6% for the comparable periods in 2014.
Research and development expenses in the fourth quarter of 2015 were $9.4 million as compared to $5.8 million in the same quarter last year. This increase is primarily due to the initiation of Phase 3 testing associated with SPN-810 during the third quarter of 2015 and the initiation of Phase 2b testing of SPN-812 during the fourth quarter of 2015.
Research and development expenses in the fourth quarter of 2015 were essentially flat compared to research and development expenses in the third quarter of 2015 as the provisioning of clinical trial material to the clinical sites was slower than we anticipated. This impacted site initiation and patient screening. R&D expenses for full-year 2015 were $29.1 million as compared to $19.6 million for full-year 2014.
This increase reflects the advancement of SPN-810 and SPN-812 into late-stage studies, including the manufacturing of clinical supplies and the screening of clinical trial sites. We continue to expect research and development expenses to increase in 2016 as clinical advancement of both SPN-810 and SPN-812 progresses.
Selling, general, and administrative expenses were $23.6 million for the fourth quarter 2015 as compared to $18 million in the same period in 2014. Selling, general, and administrative expenses were $89.2 million in 2015 as compared to $72.5 million in 2014. Higher expenses in 2015 reflect marketing, medical, and promotional programs in support of the currently commercialized products as well as work done in anticipation of launching a migraine headache indication for Trokendi XR in 2016.
For the fourth quarter, operating income totaled $6.8 million as compared to $4.7 million in the fourth quarter of 2014. Full-year 2015 operating income was $17.7 million. Excluding the impact of a $30 million royalty monetization payment, operating loss for 2014 was $5.8 million. The substantial improvement in operating income for fourth quarter and for full-year 2015 compared to the year-earlier periods reflects the strength and growth of our base business.
Net income for the fourth quarter ended December 31, 2015, was $6.9 million or $0.14 per diluted share as compared to net income of [$4.4 million] or $0.10 per diluted share in the fourth quarter of 2014. Net income for full-year 2015 was $14 million or $0.28 per diluted share as compared to net income of $19.9 million or $0.32 per diluted share for full-year 2014. Excluding the impact of the royalty monetization payment, full-year 2014 net income would have been a loss of $10.1 million.
Approximately 49.6 million weighted average diluted common shares were outstanding in the fourth quarter of 2015 and approximately 51.2 million shares for full-year 2015 as compared to 43.2 million diluted shares and 50.6 million diluted shares in the same periods for the prior year. Full-year weighted average shares outstanding take into account the dilutive impact from the convertible notes.
As of December 31, 2015, we had $117.2 million in cash, cash equivalents, marketable securities, and long-term marketable securities as compared to $94.2 million at December 31, 2014, and $101.7 million at September 30, 2015. As of March 2, 2016, approximately $6.6 million of our six-year $90 million convertible notes remain outstanding.
Turning now to annual guidance, for full-year 2016, we expect that net product sales will range from $200 million to $210 million, with operating income ranging from $28 million to $35 million. We expect R&D expense for full-year 2016 to be in the range of $55 million to $65 million as we progress late-stage development of SPN-810 and SPN-812.
I will now turn the call back to the operator for questions.
Operator
(Operator Instructions) David Amsellem, Piper Jaffray.
David Amsellem - Analyst
Thanks, just a couple. So on the migraine opportunity, can you help us think about the incremental opportunity for Trokendi? And maybe help us understand the extent to which the product is already used off-label in migraine? Does it mirror the off-label usage that we see for immediate-release topiramate in migraine?
And then secondly, can you walk us through the gross to net in the fourth quarter for both Oxtellar and Trokendi and how we should think about that trending in 2016 versus 2015? Thanks.
Jack Khattar - President and CEO
Regarding the migraine market, as we have said before, it's basically a split up -- or, I should say, the topiramate market split out to about 50%-55%, depending what data set you look at or to what point. 55% or 60% or 50% of topiramate usage is in migraine. That's regarding the market overall, in general.
And in our case, we don't have migraine on the label, obviously, and we only promote for epilepsy. But as you said, we have seen some usage from a migraine point of view because we call on neurologists for epilepsy, and neurologists know these molecules very well, clearly, and they choose (technical difficulty) whichever way they do.
So our usage has mirrored a little bit the market, again depending on when you look at it. And we don't track it very, very closely because these data sets tend to be not as predictive and not as also that reliable in general, these snapshots that you take from IMS and so forth.
So we do expect an upside from the migraine indication. To what extent that upside will be and what's the magnitude of that upside -- we are still looking at it at this point. And we are going to wait and launch the indication, and we will monitor it very quickly and see how the prescriptions are trending.
And that's why you also may recall from our previous calls, we said -- we were asked about expansion of sales force and so forth. We are considering that. We will not commit to it at this point, early on, until we see how the launch is going, how is the traction regarding the migraine, and how much upside, if we are getting any, how much is that and does it warrant the investment.
Greg Patrick - CFO
David, with respect to gross-to-net deductions, in the fourth quarter, Trokendi XR gross-to-net reductions were in the low 30%s -- let's say 30% to 32%. And Oxtellar XR in the low 50%s -- let's say 50% to 52%. You are probably aware the Company took a price increase earlier this year, and so the effect of that price increase will affect gross-to-net deductions going forward.
So we would probably expect the Oxtellar XR gross-to-net deductions because of the greater participation in Medicaid to trend a bit more in terms of percentage increase, probably into the low- to mid-50%s, and Trokendi XR would kind of be a nominal increase from where it is right now. But we wouldn't expect a great move in that regard.
David Amsellem - Analyst
That's helpful. And if I may sneak in one additional question. So just in terms of capital deployment, can you give us your latest thoughts on the aggressiveness -- your aggressiveness in seeking assets where you can leverage a sales organization? How are you thinking about that these days?
Jack Khattar - President and CEO
Nothing has really changed versus last year or the year before. We continue to be very aggressive in looking. We have several things in the hopper that we are working on. So that has not changed at all. We are also actually hoping that in general, valuations or expectations for valuations and the pricing of these assets will be a little bit more reasonable, given the changes in the marketplace and so forth. So we are very optimistic and looking forward to this year to continue to be very aggressive in pursuing these kind of things.
David Amsellem - Analyst
Okay. Thank you.
Operator
Ken Cacciatore, Cowen and Company.
Ken Cacciatore - Analyst
A question around the Trokendi XR litigation. Can you just give us an update on where things stand? I understand that you had the settlement, and now trying to get a better sense of maybe timing around discussions with other folks or actual eventual getting into trial.
And then also maybe as we continue to progress here on SPN-810, a bit of a discussion about the market opportunity itself, how you sized it or what we should look at to compare it against as we start to imagine what this could be, what this could mean to you. Thank you.
Jack Khattar - President and CEO
Yes, sure. Well, starting with the Trokendi XR litigation, as a refresher, we had initially three filers, ANDA filers: Actavis, Zydus, and Par. As some of you may recall, last year, we did settle with Par for a 2025 date. So the remaining two ANDA filers are Actavis and Zydus. And that case just went to a Markman hearing actually recently, in February, this past February.
We don't have at this point a date for the trial, so it's a little bit hard to guess. But the 30 months expiration on the product for both of these filers falls in the first quarter of 2017. I believe one of them is in February and one is like early April or end of March. So that's pretty much the timeline for that. So we expect the trial potentially to be sometime in 3Q or 4Q, depending on how things progress through the court.
Again, we have said it before that we have settled with one of the filers. Obviously, that means we are willing to settle if settlement is an option. But we also very strongly believe in our IP and we are very confident about the strategies we have, the strength of the IP, and the team that is working with us in defending these very innovative products.
So we will see how the year progresses. And we will do what we always do is try to navigate through these issues and hopefully come out on the right side of it. So we feel very strongly about the Trokendi XR case and we are looking forward to hopefully build it into a very, very long-term franchise for Supernus.
As far as SPN-810, the market is huge regarding impulsive aggression. A quick reminder from our -- back in June last year when we did the investor day, we presented some of the very, very extensive research that we did on impulsive aggression, specifically in the ADHD space, where we found prevalence as high as 32% of ADHD patients have impulsive aggression. And there is nothing out there to treat impulsive aggression that is approved by the FDA, that is clinically studied to treat that condition. And SPN-810 will be a first product approved in that space.
So we are looking at ADHD as a very, very big potential for us, followed very, very closely with areas like autism. Bipolar prevalence in autism is as high as 40%-45%, actually, of patients have impulsive aggression. In bipolar, we've seen numbers as high as 60% of patients have impulsive aggression. Patients in schizophrenia have impulsive aggression. PTSD, Alzheimer's, and so forth.
So this is a huge, wide opportunity for us. We are pioneering the whole field, starting with the innovation that we started last year in completing the validation of the diagnostic tool, the measurement of the outcome, which was agreed upon with FDA, the endpoints that the FDA agreed on.
So we are very, very excited about the potential of 810 and especially that the profile of SPN-810 compared to the existing off-label usage of products that physicians shouldn't use, but they have no choice but to use things like sedatives, where they just sedate patients, or antipsychotics that have major issues with weight gain, prolactin increases, and so forth. SPN-810 profile is a much, much better tolerability and safety. And the efficacy we saw in our Phase 2b trial was really very, very strong efficacy. So we are very, very excited about 810.
And I know you didn't ask about 812. But since I'm talking about the pipeline, 812 -- I can't tell you how excited we are since we got this new data, the safety data on 812. Because, as we all know, and most of you are familiar with ADHD, the gold standard in treating ADHD is to come up with a product that has the efficacy that is even close to a stimulant and has a better tolerability and safety profile of a non-stimulant. And we don't have that product in the marketplace today. This is the gold standard we have been looking for, for years and years.
Where stimulants are very, very effective, but they have all their issues from a controlled substance point of view, from the side effect point of view, tolerability, and so forth. And the non-stimulants that have been in the marketplace don't have great tolerability and safety profile. So we believe 812 can be that gold standard in giving you the efficacy of a stimulant or as close as possible to a stimulant, yet with a very, very clean side effects profile.
We were able to push the dose in the SAD and MAD studies to two times or three times the dosage we used in our Phase 2a study, and we showed an amazing adverse event profile versus the immediate-release formulation. So we are very, very excited about 812 as well in being potentially the gold standard in treating ADHD.
Ken Cacciatore - Analyst
Thank you.
Operator
Annabel Samimy, Stifel.
Annabel Samimy - Analyst
Thanks for taking my question. While you are on the topic of 812, are you going to be looking at the highest doses in your Phase 2b? And do you have specific designers? Is it just going to be a standard design for non-stimulant or for an ADHD trial?
Jack Khattar - President and CEO
Yes, the 812 Phase 2b trial is your typical dose-range finding study, and this is the first time we actually -- it's a pediatric study because the Phase 2a study that we did initially back a couple years ago wasn't about ADHD. So this is the first time we do it in pediatrics.
And the answer to your question is yes, we are going to push the dose up. And we are obviously at the back of getting this kind of data. We are very confident we should be able to push the dose up with minimum tolerability or safety issues. So we are really excited. It's a fairly big study: about 190-198 patients that we are targeting across several doses.
And if this study is really robust from an efficacy and tolerability profile, we believe it may be able to count as a Phase 3 study. So that's really what we are shooting for. It's a very well-designed study; a fairly sizable study. And given the profile that is emerging from 812, we are hoping that that study at some point may end up the actually the first of our Phase 3 studies.
Annabel Samimy - Analyst
Okay. How many Phase 3 studies you are going to be having, and are you going after pediatric and adult at the same time for 812?
Jack Khattar - President and CEO
Yes. Eventually we will get it for pediatric and adult. But you can't get it approved for adult, obviously, without doing the pediatric study. So the first initial program is in pediatrics. And as the data emerges over time, we plan on doing it in adult as well, absolutely.
Annabel Samimy - Analyst
Okay. And then if I can go back to migraine for a minute, I guess we've talked in the past about how the profile for an extended-relief product is more probably favorable for that prophylactic migraine type of condition because you can take it at night and it would last you through the night so that you don't have that acute migraine in the morning. Do you think, actually, that you have an opportunity to push the percentage of usage in migraine higher than what the current market is right now?
Jack Khattar - President and CEO
That is a very good point, a very good question. Personally, I believe there is an opportunity. And I will tell you one thing: if anybody can do it, our sales force can do it. These folks have done a tremendous job in building Trokendi XR. And I think if anybody can do that and really just show doctors and patients the true benefits of Trokendi XR once a day.
And you are absolutely right. It applies to migraine as much as it applied to epilepsy. Because in migraine, as you pointed out, a topiramate is used in prophylaxis. So the importance of compliance is paramount. You have to be compliant if you really want to use this product as it's designed to do to prevent and be a prophylactic treatment for migraine.
And Trokendi XR has an amazing PK profile that gives you that coverage and assurance that you will be able to use topiramate as you should from the beginning instead of what today physicians do by compromising and giving people a product that is designed to be twice a day, giving it to them once a day and telling them take it at night so you don't have to go through the side effects. And by the time you wake up in the morning, as you said, the migraine hits because you don't have enough drug in your system.
Trokendi XR -- it doesn't matter. You take it at night, you take it in the morning, whenever you want to take it. You are going to have the full 24-hour coverage for the treatment of migraines in a prophylactic manner. So it's really the ideal, ideal product for that kind of treatment. And it might be well an opportunity for us to expand the usage of topiramate in migraine, where people -- a lot -- maybe they don't use it as well because of the side effects and the issues they face.
Annabel Samimy - Analyst
Okay, great. And just one last question on 810. Was there anything in the SPA that was contrary to your expectations?
Jack Khattar - President and CEO
Oh, no. It was the sales through, yes, as we expected; no issues at all. And I think you recall we actually initiated the study even before we finalized the SPA because we didn't expect any issues. Yes.
Annabel Samimy - Analyst
Okay, great. Thank you.
Operator
Bill Tanner, Guggenheim Securities.
Bill Tanner - Analyst
Thanks for taking the question. Jack, just on the Trokendi migraine launch, it sounds like there's not going to be much of an investment, at least until you see what the uptake is like.
So just maybe a little color on that? Is this, then, really contemplated that it's going to be through the existing sales force? Or is it just going to be some awareness by other physicians that the product is out there? Just trying to understand how, maybe without investing in it from the outset, you would anticipate seeing much in the way of an uptake.
Jack Khattar - President and CEO
Yes, sure. Just to clarify, Bill, we are investing. And actually, we invested even in 2015 in the program, getting ready for the launch of the migraine indication. We are not investing specifically in expanding the sales force, but we are investing heavily from a marketing point of view, programs point of view, to launch the indication and launch the product in the migraine space.
And given that investment from a launch perspective and from marketing point of view, then when we get the label, we start promoting it to physicians. Then we will see how the traction is. We will see how the prescriptions are responding to that promotion, to the really true benefits of Trokendi XR in that space. And at that time, very close to that time, we will make decisions on expanding the sales force or not.
So the investment actually has happened and will continue to happen in the first half of this year, gearing up to the migraine launch. We were spending some serious investment behind it because we believe, as we said before, Trokendi XR together with Oxtellar XR -- but the biggest piece is Trokendi XR -- can be a $500 million franchise for us.
So we are going to make sure this opportunity -- the migraine opportunity -- is a huge opportunity for us and we are going to try to maximize it. So we are not going to skimp on any investment behind the product. But as far as headcount, as far as expansion of sales force, we will try to do it in more of a measured approach, waiting for the prescription and the trending of these prescriptions before we push the button on it.
Bill Tanner - Analyst
Got it.
Greg Patrick - CFO
So if --
Bill Tanner - Analyst
Sorry, Greg. Go ahead.
Greg Patrick - CFO
So if you would have a chance to go through our guidance, you will notice that -- and we provided guidance for R&D. It's not hard math. Then look at what the implied SG&A spend is, and you will see that on a year-over-year basis, there will be a pickup in SG&A spend.
And further to Jack's comments, that's all around the launch meeting, investing in terms of KOLs, etc. So we have a lot of marketing programs, a lot of activity with the sales force, which is really directed at the migraine launch. And that's what that spending is for.
Bill Tanner - Analyst
Got it. And that's what my follow-up was going to be, then, as it relates to the promotional efforts. Could you -- in the absence of adding salespeople, could you maybe frame that a little bit or put some color around that?
Jack Khattar - President and CEO
Yes. If you don't mind, I can't really be too specific, obviously, from a competitive point of view. So I'll just leave it that way.
Bill Tanner - Analyst
Okay. And then maybe if I could do one follow-up. Jack, you mentioned that the Company is looking for other assets. It seems like you are very enthusiastic about prospects for 810 and 812, and those would come online a couple of years hence. But I guess trying to gauge what's the real level of activity or the desire to do something, because it certainly looks like both of those product could be pretty significant in the out years.
Jack Khattar - President and CEO
Oh, absolutely. Look, from a strategy point of view and our vision in building Supernus, we've said it several times. We are a little bit different as a profile, as a specialty pharma Company. We're different than several other companies out there who have depended historically on buy and sell instead of develop and truly have an R&D engine that we have.
So we have an ideal combination of products in the market that are very young products, only three years or two years into the marketplace, that can give us tremendous growth just by maximizing their potential. And as you rightly pointed out, we have two products that are not really far off -- we're already in 2016, and we could be launching 810 in 2019, maybe earlier. We will see how the program comes out.
So you are absolutely right; we have a very strong engine that will give us the lot of future potential for this Company, a lot of growth potential, given the opportunity of 810 that I went through and the amazing profile that 812 seems to be looking like.
So we never felt that we are really desperate, so to speak, or have to do a business development and if we don't, we'll never survive as a company. That's never the case with us.
However, however, we are after building Supernus in any means that really makes sense to us strategically and at a reasonable price. So we haven't been willing to pay the exorbitant prices for assets that have available for years. Over the last two or three years, people have been overpaying.
As I said earlier, if these prices become a little bit more reasonable, sellers' expectations as far as valuations, they come down to earth a little bit than they were historically, we definitely will have a better shot at doing these kind of things. Because we are not going to do something just to be dilutive and do something just because people -- or some people, I should say -- want us to do something.
Bill Tanner - Analyst
All right.
Jack Khattar - President and CEO
It has to make sense financially. It has to make sense strategically. And to your question, we have been very, very active and we will continue to be active -- I mean, active not just looking and sniffing around. No, we have been making bids, we have been making offers, we have been making due diligences on certain products and so forth. But then, at the end of the day, as you guys know, in corporate development, you can never guess exactly the timing of these things because a lot of things have to fall in place.
Bill Tanner - Analyst
Got it. Okay, all right. Thanks for the color.
Operator
John Boris, SunTrust.
John Boris - Analyst
Thanks for taking the questions. Jack, Greg, congratulations on the 2015 accomplishments. First question -- if you look at your operating margin range of $28 million to $35 million, Street is currently at $43 million. Differential there from the midpoint to the upper end is about $8 million to $10 million. That's a fairly substantial amount of potential investment which you've articulated is going to go into chronic migraine.
Can you elaborate as to whether you are going to use a contract sales organization, so if it's potentially successful, you could fold in 20-30 reps that could be concentrating on the migraine launch?
And then secondly, when you look at the product concept in chronic migraine, what percent of patients currently take topiramate once daily to treat chronic migraine? I understand it could be a substantial amount. And then what have physicians said about your product concept? What are the attributes that are going to get them to potentially prescribe in chronic migraine relative to once- or twice-daily topiramate?
And then I have one additional follow-up.
Jack Khattar - President and CEO
Yes. Regarding the contract sales force, that is not what the culture of Supernus is. We tend to not like doing contract sales force because we want our employees, our sales reps to feel that Supernus is not a company they work for, Supernus is their Company. And that's the culture we have across the Company overall, not just the sales force, but every employee at Supernus. We all work together.
So we are not too much -- we're not favorable to things like contract sales forces. So if anything, if we expand the sales force, it will be our own sales force, our own culture, our own people promoting the product.
As far as the current usage of topiramate in migraine once a day, in general, they don't break it up really by indication. But if you look at the topiramate market in general, about 40% or 35% to 40% of topiramate prescriptions are once a day. Okay? And you have to believe a lot of it or most of it is probably in the migraine setting, as we see in the field and we see from physicians. So that's about the percentages you could look at it.
But again, they are using it -- they are compromising the therapy and they are using a product that is not designed to be a once-a-day product just to get around the side effects -- cognition, dizziness, you name it -- so that people don't feel dopey in the morning when they take Topamax in the morning. So they tell them take it in the evening so you sleep through these side effects.
Well, obviously, with Trokendi XR, you don't have these issues. And that's really the true, true benefit of Trokendi XR once a day in migraine as well as obviously in epilepsy, which we have talked about in the last couple of years. So the benefits of Trokendi XR in migraine are probably as good as they are in epilepsy, if not even better.
Because migraine again, folks, and I'll remind people: migraine is a serious condition as well. People who suffer from migraines, some of them have to be locked up a day or two days at home without seeing the sunlight or what have you. And they are not really functional until that migraine goes away. So if you have a way to prevent migraine or use it in a prophylactic way, that is truly a change in their lifestyle. So we think Trokendi XR can be very, very powerful as far as delivering very strong benefits to patients and physicians in that space.
John Boris - Analyst
So next question -- thanks for that; that's very helpful. And then next question just has to do with your strategic assets here in Trokendi XR and Oxtellar and the patent life that you are potentially afforded on these assets.
You obviously selected chronic migraine as a use that the product has. It also is used quite extensively in psychiatric indications, like bipolar disorder. If you have date-certain launches in April of 2025, why wouldn't you file for potential psychiatric uses for Trokendi XR?
And then same question for Oxtellar XR: if you look at trigeminal neuralgias, if you look at bipolar use, it's pretty extensive with Oxtellar XR. So if you're looking to expand into psychiatry, why not file these additional assets, especially since they have longer patent life, in the psychiatry area that would then complement your thrust into those areas?
Jack Khattar - President and CEO
Yes. Topiramate -- you are absolutely right; it is used in psychiatry. But it's not indicated for any of these psychiatry uses. So even Topamax itself does not have any psychiatric conditions that are part of its label. So if we were or anybody or any other company were to add psychiatry to its label, you will have to go through the clinical program and develop these programs.
We are not saying we will never do that. So I'm not going to speak about this at this point. But certainly once we have assurance and we have strong confidence and conviction in what we believe the long lifecycle of these products will be, we will invest at the appropriate time to make sure we'll get the best growth behind these products.
Similarly, oxcarbazepine is only indicated in epilepsy. However, Trileptal clearly for many, many years was used off-label in psychiatry for bipolar disorder. Again, for us to be able to promote Oxtellar XR or add it to the label, we will have to undergo a full, full bipolar program behind our product, which is not a cheap thing to do. These programs can be very, very expensive. If you want to do two positive Phase 3 studies in bipolar, that's not a very small investment at all.
So we look at all these opportunities, actually, as we weigh our pipeline, we weigh the opportunities and the potential return on investment. And that's a continuous process. We will continue to evaluate all that, all the time, as we clear the generic hurdles and we ensure that we have the lifecycle for these two products.
John Boris - Analyst
Thanks for that. Can I just slip one more in on SPN-810? One area of pushback has to do with IP. Can we just get an update on what your IP position is behind SPN-810?
Jack Khattar - President and CEO
Yes. For SPN-810, we have several, several layers on the IP side. Although the molecule is a very old molecule, this will be novel use. So we have applications as far as the indication itself as a novel use in impulsive aggression. The other IP we have is on the formulation itself and the controlled release, extended release, which we have been very successful historically in creating very good IP. This is a truly innovative product.
We also have said that we are creating IP on the synthesis of the API itself, because not too many people even make molindone hydrochloride and we were able to innovate around the synthesis process for that API. And then the last point, which we made also, is that we already have in place a second-generation follow-on program for SPN-810 before we even launched the first product. So we intend to have this franchise well into the 2030s because all our patents expire in 2029 or even beyond to 2033.
John Boris - Analyst
Very helpful. Thank you. And again, congrats on the results.
Operator
David Buck, Northland Capital.
David Buck - Analyst
Thanks for taking the question. First one for Jack -- in the R&D target this year, was there anything in terms of acceleration of spend or acceleration of the trials for SPN-812, in particular? And can you talk a little bit about the pace of enrollment that you would expect for both SPN-810 and 812?
And just for Greg, can you talk a little bit about whether you are expecting any changes that we should be aware of in gross margin for 2016, and what's the best sense on when you would start to book a tax rate? You are booking a nominal tax rate again this year. What's the best estimate for when you would be booking a full tax rate? Thanks.
Jack Khattar - President and CEO
Yes, sure. Regarding 810, David, as Greg mentioned in his commentary earlier, we had a slow start because we had slight delays over the holidays at year end with the clinical material and the clinical supplies for the trial from our vendors. And because of that, we are actually starting the patient dosing this quarter versus we were expecting it to be end of December, which we realized even at that time maybe a tough period with the holidays. But that was the initial target we have.
However, that hasn't delayed the trial, so to speak. But it's shifted spending from an R&D point of view from 2015 to 2016. And these are things -- sometimes you can control some of these things, sometimes you can't. And these shifts sometimes -- they are all in million dollars. They are not like a $200,000 or $300,000.
And that's why unfortunately, you see the R&D spending over time -- it's a little bit hard for us to predict on a quarterly basis or even on an annual basis, as happened last year, where things shifted a little bit into 2016.
Some of you may recall on 2015, we had talked very at length about R&D being about 50% more than 2014, and it came in exactly around that range. But we did talk about sometimes that the R&D spending in 2015 could have been $33 million, $35 million. But we ended up around $30 million.
So naturally, that money, which we could have spent in 2015, ended up sliding into 2016, and therefore the R&D number in 2016 is higher probably than some people expected. But we control what we can control, obviously. We can't control vendors all the time or holidays, and that's why we try to always caution people that this is a moving, sliding issue, especially when you have two programs running at such a full speed, with two Phase 3s and one Phase 2b trial. That's a lot of R&D that is going on, and naturally things are going to shift around all the time.
As far as enrollment on 812, it's a little bit actually ahead of what we have expected so far versus 810 was a little bit behind, couple weeks behind. So again, these things are going to shift all the time. And we will definitely give you guys update on enrollment and so forth throughout the year as we get closer to meaningful updates. Not, now we have 15 patients and tomorrow we have 25 patients, but something which is meaningful milestones through these trials to help you and guide you a little bit better on the R&D spending and so forth.
So Greg?
Greg Patrick - CFO
David, you asked about gross margin. Gross margins have been very stable over the past several years. If anything, we would expect them to possibly improve a bit going forward, simply because the cost increases that we are experiencing through the manufacturers that we are dealing with, our partners, are less than in overall the price increase for the product. So it could trend northward of 94%, but I think it's going to be a modest trend. And you will see that only with the fullness of time.
You asked about tax rates. That's an interesting question and a complicated question. Right now, we are not a 40% tax-rate-paying entity. Our NOLs at the end of the year, end of 2015, were pushing $100 million. As we look forward to when we become a fully taxpaying customer, it's driven by two things, of course. First of all, what we think the operating income is going to grow, going forward. And we've provided our guidance in that regard this morning.
And the second thing then has to do with how we can utilize the NOLs going forward. There are restrictions in the IRS code in terms of how quickly we can push them or utilize them over time. We expect to utilize them completely, but we might not be able to utilize them so that we can zero out or drive our tax rate close to zero over the next couple of years.
That being said, my expectation would be that somewhere in the 2018 time frame, we will push through our NOLs and will be very, at least for part of the year, a full statutory tax rate. So I think the next couple of years we will see kind of a modest tax rate, but 2018-2019 I think we will have burned off the NOLs.
David Buck - Analyst
Okay.
Operator
Thank you. And that concludes our question-and-answer session for today. I would like to turn the conference back over to Jack Khattar for any closing comments.
Jack Khattar - President and CEO
Thank you. In summary, 2015 was another stellar year for Supernus. Our commercial team continued to grow Trokendi XR and Oxtellar XR, delivering strong product sales during the year. And our R&D organization made significant progress as they advanced our pipeline products through clinical development and into late-stage clinical testing. The mid-point of our 2016 guidance reflects very strong and healthy growth of 78% in operating income and 43% in net product sales, while at the same time doubling our investment in R&D.
We are an emerging growth company that will continue to invest in its future while at the same time deliver consistent and strong growth and profitability. We look forward to updating you through the year on our progress in the clinical trials and to making 2016 yet another remarkable year for Supernus. Thanks for joining us this morning.
Operator
Thank you. Ladies and gentlemen, thank you for your participation in today's conference. This does conclude the program and you may now disconnect. Everyone have a good day.