Common stock offered

8,372,000 shares

Common stock to be outstanding after this offering

23,678,190 shares

Over-allotment option

1,255,800 shares

Initial public offering price

$15.00 per share

Net proceeds

We estimate that the net proceeds from our issuance and sale of 8,372,000 shares of our common stock in this offering will be approximately $114.2 million, based on the initial public offering price of $15.00 per share, after deducting underwriting discounts and commissions and estimated offering expenses payable by us. If the underwriters exercise in full their over-allotment option, we estimate that the net proceeds from this offering will be approximately $131.7 million.

NASDAQ Global Select Market Symbol

PTCT

Pro forma as adjusted financial data

Giving effect to this offering, as of March 31, 2013, our pro forma as adjusted balance sheet data would have been as follows (in thousands):

Cash, cash equivalents and short-term investments

$

168,911

Working capital

143,410

Total assets

178,570

Long-term debt, including current portion

3,835

Convertible preferred stock

—

Accumulated deficit

(291,909

)

Total stockholders’ (deficit) equity

148,242

Giving effect to this offering, as of March 31, 2013, our pro forma as adjusted total capitalization would have been approximately $152.1 million

Giving effect to this offering, our pro forma net tangible book value as of March 31, 2013 would have been $148.2 million, or $6.26 per share. This represents an immediate increase in pro forma net tangible book value per share of $4.03 to existing investors and immediate dilution of $8.74 in pro forma net tangible book value per share to new investors purchasing common stock in this offering

Potential purchases by existing principal stockholders or their affiliates and entities associated with certain of our directors

Brookside Capital Partners Fund, L.P., HBM Healthcare Investments (Cayman) Ltd., Section Six Partners, L.P., Longwood Fund, L.P., and Vulcan Ventures Incorporated, which are existing principal stockholders of ours or their affiliates and entities affiliated with certain of our directors, have indicated an interest in purchasing an aggregate of 3,250,000 shares of our common stock in this offering at the initial public offering price for an aggregate purchase price of approximately $48.75 million in the following amounts: Brookside Capital Partners Fund, L.P.: $18.75 million or 1,250,000 shares; HBM Healthcare Investments (Cayman) Ltd.: $10.0 million or 666,667 shares; Section Six Partners, L.P.: $8.0 million or 533,333 shares; Longwood Fund, L.P.: $7.0 million or 466,667 shares; and Vulcan Ventures Incorporated: $5.0 million or 333,333 shares. However, because indications of interest are not binding agreements or commitments to purchase, these stockholders may determine to purchase fewer shares than they indicate an interest in purchasing or not to purchase any shares in this offering.