Filed pursuant to Rule 424(b)(3)
(Registration Statement No. 333-219676
22,112,775 Shares of Common Stock
This prospectus relates to the possible resale, from time to time,
by the selling stockholders identified in this prospectus of up to 22,112,775 shares of our common stock, par value $0.001 per
share, issuable upon conversion of our Series A Convertible Preferred Stock (subject to adjustment as set forth in the Certificate
of Designation of Preferences, Rights and Limitations of Series A Convertible Preferred Stock), initially issued in a private placement,
which closed on August 2, 2017. We are not selling any shares of common stock under this prospectus and will not receive any of
the proceeds from the sale of shares of common stock by the selling stockholders.
The selling stockholders may offer the shares from time to time
as each selling stockholder may determine through public or private transactions or through other means described in the section
entitled “Plan of Distribution” or a supplement to this prospectus. Each selling stockholder may also sell shares under
Rule 144 under the Securities Act of 1933, as amended, if available, rather than under this prospectus.
We are registering the offer and sale of these shares pursuant to
certain registration rights granted to the selling stockholders. The registration of these shares of common stock does not necessarily
mean that any of the shares will be offered or sold by the selling stockholders. The timing and amount of any sale is within the
sole discretion of each selling stockholder.
The selling stockholders will pay all underwriting discounts and
selling commissions, if any, in connection with the sale of the shares of common stock. We have agreed to pay certain expenses
in connection with this registration statement and to indemnify the selling stockholders against certain liabilities. To our knowledge,
as of the date of this prospectus, no underwriter or other person has been engaged to facilitate the sale of shares of common stock
in this offering.
Our common stock is listed on The NASDAQ Global Market under the
symbol “PRTO.” On August 11, 2017, the closing price of our common stock was $1.30 per share.
Investing in our common stock involves a high degree of risk.
You should carefully read the risks and uncertainties included herein under the heading “Risk Factors” on page 7 of
this prospectus, and under similar headings in our Annual Report on Form 10-K for the fiscal year ended December 31, 2016 and our
Quarterly Reports on Form 10-Q for the quarterly periods ended March 31, 2017, which have been filed with the Securities and Exchange
Commission, or the SEC, and are incorporated by reference in this prospectus and in the other documents that are filed after the
date hereof and incorporated by reference into this prospectus.
Neither the Securities and Exchange Commission nor any state
securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus.
Any representation to the contrary is a criminal offense.
The date of this prospectus is August 21, 2017.