In December 2013, we issued warrants to purchase 1,420,250 shares of our common stock (the “2013 Warrants”) in a firm commitment underwritten public offering. The 2013 Warrants were exercisable from the date of the original issuance and will expire on December 16, 2018. The 2013 Warrants contain certain anti-dilution protections, as more fully described below, and have an exercise price of $3.57 as of the date of this prospectus. The offering and sale of the 2013 Warrants, and the shares of common stock underlying the 2013 Warrants, were registered under the Prior Registration Statement.

In August 2014, we issued warrants to purchase 2,000,000 shares of our common stock (the “2014 Warrants”) in a firm commitment underwritten public offering. The 2014 Warrants were exercisable from the date of the original issuance and will expire on August 5, 2019. The 2014 Warrants contain certain anti-dilution protections, as more fully described below, and have an exercise price of $2.51 as of the date of this prospectus. The offering and sale of the 2014 Warrants, and the shares of common stock underlying the 2014 Warrants, were registered under the Prior Registration Statement.

In February 2015, we issued Series A warrants to purchase 2,216,667 shares of our common stock (the “Series A Warrants”) in a firm commitment underwritten public offering. The Series A Warrants were exercisable from the date of the original issuance and will expire on February 3, 2020. The Series A Warrants contain certain anti-dilution protections, as more fully described below, and have an exercise price of $0.30 as of the date of this prospectus. The offering and sale of the Series A Warrants, and the shares of common stock underlying the Series A Warrants, were registered under the Prior Registration Statement.

In May 2015, we issued Series C warrants to purchase 430,000 shares of our common stock (the “Series C Warrants”) in a firm commitment underwritten public offering. The Series C Warrants were exercisable from the date of the original issuance and will expire on May 19, 2020. The Series C Warrants contain certain anti-dilution protections, as more fully described below, and have an exercise price of $1.84 as of the date of this prospectus. The offering and sale of the Series C Warrants, and the shares of common stock underlying the Series C Warrants, were registered under the Prior Registration Statement.

In December 2015, we issued Series D warrants to purchase 10,050,000 shares of our common stock (the “Series D Warrants”) in a firm commitment underwritten public offering. The Series D Warrants were exercisable beginning on June 11, 2016 and will expire on December 11, 2020. The Series D Warrants have an exercise price of $0.10 as of the date of this prospectus and the holders of Series D Warrants are not entitled to any further anti-dilution adjustments. The offering and sale of the Series D Warrants, and the shares of common stock underlying the Series D Warrants, were registered under the Prior Registration Statement.

In April 2016, we issued Series F warrants to purchase 10,292,858 shares of our common stock (the “Series F Warrants”) and Series H Warrants to purchase 20,585,716 shares of our common stock (the “Series H Warrants” and, together with the 2013 Warrants, the 2014 Warrants, the Series A Warrants, the Series C Warrants, the Series D Warrants and the Series F Warrants, the “Warrants”) in a firm commitment underwritten public offering. The Series F Warrants will be exercisable beginning on October 1, 2016 and will expire on April 1, 2021. The Series F Warrants contain certain anti-dilution protections, as more fully described below, and have an exercise price of $0.30 as of the date of this prospectus. The Series H Warrants were exercisable from the date of the original issuance, will expire on October 1, 2016 and have an exercise price of $0.75 per share as of the date of this prospectus. The offering and sale of the Series F Warrants and the Series H Warrants, and the shares of common stock underlying the Series F Warrants and the Series H Warrants, were registered under the Prior Registration Statement.

The Company has the right at any time during the term of the Series D Warrants, Series F Warrants or Series H Warrants to reduce the then-existing exercise price of the Series D Warrants, Series F Warrants or Series H Warrants, respectively, to any amount and for any period of time deemed appropriate by our board of directors.

As of the date of this prospectus, the following number of each series of Warrants remain unexercised and outstanding:

Exercise Mechanics

The Warrants may be exercised by delivering to the Company and/or the warrant agent, as applicable, a written notice of election to exercise, appropriately completed, duly signed and delivered, and delivering to the Company cash payment of the exercise price, if applicable. Upon delivery of the written notice of election to exercise, appropriately completed and duly signed, and cash payment of the exercise price, if applicable, on and subject to the terms and conditions of the Warrant or warrant agreement governing such series of Warrants, as applicable, we will deliver or cause to be delivered, to or upon the written order of such holder, the number of whole shares of common stock to which the holder is entitled, which shares may be delivered in book-entry form. No fractional shares will be issued upon exercise of the Warrants. If a Warrant is exercised for fewer than all of the shares of common stock for which such warrant may be exercised, then upon request of the holder and surrender of such Warrant, we may issue a new Warrant or warrant certificate exercisable for the remaining number of shares of common stock.

If, and only if, a registration statement relating to the issuance of the Warrant Shares underlying any series of the Warrants is not then effective or available, a holder of such series of Warrants may exercise such Warrants on a cashless basis, where the holder receives the net value of such Warrants in shares of common stock. However, if an effective registration statement is available for the issuance of the shares underlying a series of Warrants, a holder of such series of Warrants may only exercise such Warrants through a cash exercise. Shares issued pursuant to a cashless exercise would be issued pursuant to the exemption from registration provided by Section 3(a)(9) of the Securities Act, and thus the shares of common stock issued upon such cashless exercise would take on the characteristics of the Warrants being exercised, including, for purposes of Rule 144(d) promulgated under the Securities Act, a holding period beginning from the original issuance date of such Warrants.

If we fail to timely deliver shares of common stock pursuant to any exercise of any Series D Warrant, Series F Warrant or Series H Warrant, and such exercising holder elects to purchase shares of common stock (in an open market transaction or otherwise) to deliver in satisfaction of a sale by such holder of all or a portion of the shares of common stock for which such Warrant was exercised, then we will be required to deliver, at the holder’s election, either (i) an amount in cash equal to the full purchase price paid by the holder to acquire such alternative shares or (ii) (A) the shares of common stock for which the Series D Warrant, Series F Warrant or Series H Warrant, as applicable, was exercised and (B) an amount in cash equal to the excess (if any) by which the price paid for the alternative shares exceeds the lowest closing sale price of our common stock during the period beginning on the exercise date and ending on the date such payment is delivered. Notwithstanding the foregoing, if we are prohibited by restrictions contained in any of our credit agreements from making any payments described above in cash, we may instead satisfy any such payment obligation by delivering to the holder a number of shares of common stock equal to the cash payment amount divided by 90% of the last volume weighted average price of our common stock on the date of such payment.

Fundamental or Extraordinary Transactions

If, at any time while the Warrants are outstanding, we directly or indirectly, in one or more related transactions, enter into a “fundamental transaction” or “extraordinary transaction”, as described in the applicable Warrants or warrant agreements and generally including any merger with or into another entity, sale of all or substantially all of our assets, tender offer or exchange offer, or reclassification of our common stock, then each holder shall become entitled to receive the same amount and kind of securities, cash or property as such holder would have been entitled to receive upon the occurrence of such fundamental or extraordinary transaction if the holder had been, immediately prior to such fundamental or extraordinary transaction, the holder of the number of shares of common stock then issuable upon exercise of such holder’s Warrants. Any successor to us, surviving entity or the corporation purchasing or otherwise acquiring such assets shall assume the obligation to deliver to the holder such alternate consideration, and the other obligations, under the Warrants.

Additionally, in the event of an extraordinary transaction in which the 2013 Warrants, 2014 Warrants, Series A Warrants or Series C Warrants are assumed by a company that is not a publicly traded company, we or any successor entity will pay at the option of any holder of such Warrants, exercisable at any time concurrently with or within 30 days after the consummation of the extraordinary transaction, an amount of cash equal to the value of such Warrants as determined in accordance with the Black Scholes option pricing model and the terms of such Warrants.

In the event of a fundamental transaction, we or the successor entity will, at the option of any holder of Series D Warrants, Series F Warrants or Series H Warrants, exercisable prior to the date that is 30 days after the public announcement of the consummation of the fundamental transaction, purchase the unexercised portion of such Warrants for an amount of cash equal to the value of such Warrants as determined in accordance with the Black Scholes option pricing model and the terms of such Warrants.

Anti-Dilution Adjustments

The exercise price and the number and type of securities purchasable upon exercise of the Warrants are subject to adjustment upon certain corporate events, including certain combinations, consolidations, liquidations, mergers, recapitalizations, reclassifications, reorganizations, stock dividends and stock splits, a sale of all or substantially all of our assets and certain other events.

The 2013 Warrants, 2014 Warrants and Series C Warrants contain broad-based weighted average anti-dilution protection upon the issuance of any common stock, securities convertible into common stock or certain other issuances at a price below the then-existing exercise price of such Warrants, with certain exceptions; provided, however, that in no event will the exercise price of the 2013 Warrants, 2014 Warrants or Series C Warrants be less than $0.10 per share.

The Series A Warrants and Series F Warrants contain full ratchet anti-dilution protection upon the issuance of any common stock, securities convertible into common stock or certain other issuances at a price below the then-existing exercise price of the Series A Warrants and/or Series F Warrants, as applicable, with certain exceptions; provided, however, that in no event shall the exercise price of the Series F Warrants be less than $0.10 per share (as adjusted for stock splits, stock dividends, stock combinations, recapitalizations and similar events).

The terms of the Warrants, including the anti-dilution protections described above, may make it difficult for us to raise additional capital at prevailing market terms in the future.

Rights as a Stockholder

Except as set forth in the respective Warrants, the Warrants do not confer upon holders any voting or other rights as stockholders of the Company.

If, at any time while the Series D Warrants, Series F Warrants or Series H Warrants are outstanding, we declare or make any dividend or other distribution of our assets to holders of shares of our common stock, by way of return of capital or otherwise (including, without limitation, any distribution of cash, stock or other securities, property, options, evidence of indebtedness or any other assets by way of a dividend, spin off, reclassification, corporate rearrangement, scheme of arrangement or other similar transaction) or we grant, issue or sell any options, convertible securities or rights to purchase stock, warrants, securities or other property pro rata to the record holders of any class of common stock (in each case, “Distributed Property”), then each holder of a Series D Warrant, Series F Warrant or Series H Warrant shall receive, with respect to the shares of common stock issuable upon exercise of such Series D Warrant, Series F Warrant or Series H Warrant, the Distributed Property that such holder would have been entitled to receive had the holder been the record holder of such number of shares of common stock issuable upon exercise of the Warrant immediately prior to the record date for such Distributed Property.

Ownership Limitation

Any exercise notice with respect to 2013 Warrants, 2014 Warrants, Series A Warrants or Series C Warrants delivered by a holder thereof will be deemed automatically not to have been so delivered by such holder to the extent, but only to the extent, that delivery of shares of our common stock or any other security otherwise deliverable upon such exercise

would result in such holder having a “beneficial ownership,” as determined in accordance with Section 13(d) of the Exchange Act and the rules thereunder (“Beneficial Ownership”), of our common stock or any other class of registered equity security (a “Class”) in excess of 19.999% of the number of outstanding shares of our common stock or such Class (the “19.999% Ownership Limitation”). Any purported delivery to any holder shall be void and have no effect to the extent, but only to the extent, that after such delivery, such holder would have Beneficial Ownership of our common stock or any such Class in excess of the 19.999% Ownership Limitation.

Notwithstanding the foregoing, during any period of time in which the Beneficial Ownership of our common stock by any holder of 2013 Warrants, 2014 Warrants, Series A Warrants or Series C Warrants is less than 10%, any exercise notice with respect to 2013 Warrants, 2014 Warrants, Series A Warrants or Series C Warrants delivered by such a holder will be deemed automatically not to have been so delivered by such holder to the extent, but only to the extent, that delivery of shares of our common stock or any other security otherwise deliverable upon such exercise would result in such holder having a Beneficial Ownership of our common stock or any other Class in excess of 9.999% of the number of outstanding shares of our common stock or such Class (the “9.999% Ownership Limitation”). Any purported delivery to any such holder shall be void and have no effect to the extent, but only to the extent, that after such delivery, such holder would have Beneficial Ownership of our common stock or any such Class in excess of the 9.999% Ownership Limitation.

Notwithstanding the foregoing, during any period of time in which the Beneficial Ownership of our common stock by any holder of 2013 Warrants, 2014 Warrants, Series A Warrants or Series C Warrants is less than 5%, any exercise notice with respect to 2013 Warrants, 2014 Warrants, Series A Warrants or Series C Warrants delivered by such a holder will be deemed automatically not to have been so delivered by such holder to the extent, but only to the extent, that delivery of shares of our common stock or any other security otherwise deliverable upon such exercise would result in such holder having a Beneficial Ownership of our common stock in excess of 4.999% of the number of outstanding shares of our common stock or such Class (the “4.999% Ownership Limitation”). Any purported delivery to any such holder whose Beneficial Ownership of our common stock is less than 5% shall be void and have no effect to the extent, but only to the extent, that after such delivery, such holder would have Beneficial Ownership of our common stock or any such Class in excess of the 4.999% Ownership Limitation.

By written notice to us, any holder of 2013 Warrants, 2014 Warrants, Series A Warrants or Series C Warrants may from time to time increase or decrease either or both of the 9.999% Ownership Limitation or the 4.999% Ownership Limitation to any other percentage not in excess of the 19.999% Ownership Limitation; provided that any such increase will not be effective until the 61st day after such notice is delivered to us.

Any exercise notice with respect to Series D Warrants, Series F Warrants or Series H Warrants delivered by a holder thereof will be deemed automatically not to have been so delivered by such holder to the extent, but only to the extent, that delivery of shares of our common stock or any other security otherwise deliverable upon such exercise would result in such holder having Beneficial Ownership in excess of the 4.999% Ownership Limitation. Any purported delivery to any holder shall be void and have no effect to the extent, but only to the extent, that after such delivery, such holder would have Beneficial Ownership of our common stock or any such Class in excess of the 4.999% Ownership Limitation.

By written notice to us, any holder of Series D Warrants, Series F Warrants or Series H Warrants may increase or decrease this beneficial ownership limitation to any other percentage not in excess of 9.99%; provided that any such increase will not be effective until the 61st day after such notice is delivered to us.

For purposes of calculating Beneficial Ownership for each of the immediately six preceding paragraphs, the aggregate number of shares of our common stock beneficially owned by a holder will include the number of shares of our common stock issuable upon exercise of the Warrants with respect to which the determination of such sentence is being made, but shall exclude the number of shares of our common stock which are issuable upon (i) exercise of the remaining, unexercised Warrants beneficially owned by such holder, and (ii) exercise or conversion of the unexercised or unconverted portion of any of our other securities beneficially owned by such holder (including, without limitation, any convertible notes, convertible stock or warrants) that are subject to a limitation on conversion or exercise analogous to the limitation contained herein.

These ownership limitations will be construed, corrected and implemented in a manner so as to effectuate the intended beneficial ownership limitations herein contained and the shares of our common stock underlying the Warrants in excess of the 19.999% Ownership Limitation, 9.999% Ownership Limitation or the 4.999% Ownership Limitation will not be deemed to be beneficially owned by a holder for any purpose including for purposes of Section 13(d) or Rule 16a-1(a)(1) of the Exchange Act.

CONTENTS​LEGAL OWNERSHIP OF SECURITIES

We can issue securities in registered form or in the form of one or more global securities. We describe global securities in greater detail below. We refer to those persons who have securities registered in their own names on the books that we or any applicable trustee, depositary or warrant agent maintain for this purpose as the “holders” of those securities. These persons are the legal holders of the securities. We refer to those persons who, indirectly through others, own beneficial interests in securities that are not registered in their own names, as “indirect holders” of those securities. As we discuss below, indirect holders are not legal holders, and investors in securities issued in book-entry form or in street name will be indirect holders.

Book-Entry Holders

We may issue securities in book-entry form only, as we will specify in the applicable prospectus supplement. This means securities may be represented by one or more global securities registered in the name of a financial institution that holds them as depositary on behalf of other financial institutions that participate in the depositary’s book-entry system. These participating institutions, which are referred to as participants, in turn, hold beneficial interests in the securities on behalf of themselves or their customers.

Only the person in whose name a security is registered is recognized as the holder of that security. Securities issued in global form will be registered in the name of the depositary or its participants. Consequently, for securities issued in global form, we will recognize only the depositary as the holder of the securities, and we will make all payments on the securities to the depositary. The depositary passes along the payments it receives to its participants, which in turn pass the payments along to their customers who are the beneficial owners. The depositary and its participants do so under agreements they have made with one another or with their customers; they are not obligated to do so under the terms of the securities.

As a result, investors in a book-entry security will not own securities directly. Instead, they will own beneficial interests in a global security, through a bank, broker or other financial institution that participates in the depositary’s book-entry system or holds an interest through a participant. As long as the securities are issued in global form, investors will be indirect holders, and not legal holders, of the securities.

Street Name Holders

We may terminate a global security or issue securities in non-global form. In these cases, investors may choose to hold their securities in their own names or in “street name.” Securities held by an investor in street name would be registered in the name of a bank, broker or other financial institution that the investor chooses, and the investor would hold only a beneficial interest in those securities through an account he or she maintains at that institution.

For securities held in street name, we will recognize only the intermediary banks, brokers and other financial institutions in whose names the securities are registered as the holders of those securities, and we will make all payments on those securities to them. These institutions pass along the payments they receive to their customers who are the beneficial owners, but only because they agree to do so in their customer agreements or because they are legally required to do so. Investors who hold securities in street name will be indirect holders, not legal holders, of those securities.

Legal Holders

Our obligations, as well as the obligations of any applicable trustee and of any third parties employed by us or a trustee, run only to the legal holders of the securities. We do not have obligations to investors who hold beneficial interests in global securities, in street name or by any other indirect means. This will be the case whether an investor chooses to be an indirect holder of a security or has no choice because we are issuing the securities only in global form.

For example, once we make a payment or give a notice to the legal holder, we have no further responsibility for the payment or notice even if that legal holder is required, under agreements with depositary participants or customers or by law, to pass it along to the indirect holders but does not do so. Similarly, we may want to obtain the approval of the legal holders to amend an indenture, to relieve us of the consequences of a default or of our obligation to comply with a particular provision of the indenture or for other purposes. In such an event, we would seek approval only from the legal holders, and not the indirect holders, of the securities. Whether and how the legal holders contact the indirect holders is up to the legal holders.

Special Considerations For Indirect Holders

If you hold securities through a bank, broker or other financial institution, either in book-entry form or in street name, you should check with your own institution to find out: