In April 2017, the Company entered into a Controlled Equity Offering Sales Agreement (the “Sales Agreement”) with Cantor Fitzgerald & Co. (“Cantor”), as sales agent, pursuant to which the Company had the right to offer and sell, from time to time, through Cantor, shares of the Company’s common stock, par value $0.001 per share, by any method deemed to be an “at the market offering” as defined in Rule 415 under the Securities Act of 1933, as amended (the “Cantor ATM Offering”). In April 2017, the Company also filed a prospectus supplement (File No. 333-216977) with the SEC relating to the offer and sale of up to $20.0 million of common stock in the Cantor ATM Offering. In June 2017, the Company filed a first amendment to the prospectus supplement with the SEC to increase the amount of common stock that could be offered and sold in the Cantor ATM Offering under the Sales Agreement to $40.0 million in the aggregate, inclusive of the common stock previously sold in the Cantor ATM Offering prior to the date of the first amendment. In January 2018, the Company filed a second amendment to the prospectus supplement with the SEC to decrease the amount of common stock that could be offered and sold in the Cantor ATM Offering under the Sales Agreement to $23.0 million in the aggregate, inclusive of the common stock sold in the Cantor ATM Offering prior to the date of the second amendment. In February 2018, the Company and Cantor mutually agreed to terminate the Sales Agreement.

Prior to termination of the Sales Agreement, the Company sold 5,733,314 shares of common stock under the ATM Offering at then-market prices for total gross proceeds of approximately $21.1 million, including 261,352 shares of common stock sold for gross proceeds of $0.6 million during the first quarter ended March 31, 2018. After $0.6 million of sales commissions and $0.2 million of other offering expenses paid by the Company in connection with the ATM Offering, the Company’s aggregate net proceeds from the ATM Offering were approximately $20.2 million. Sales commissions and offering expenses have been recorded as a reduction of proceeds received in arriving at the amount recorded in additional paid-in capital in the accompanying balance sheets as of December 31, 2018. There were no proceeds from the ATM Offering for the quarter ended March 31, 2019.

In January 2018, the Company completed an underwritten public offering of 13,915,000 shares of its common stock at a price of $2.90 per share, including 1,815,000 shares sold pursuant to the exercise in full of the underwriters’ option to purchase additional shares. The Company sold its common stock through an underwriting agreement with Leerink Partners LLC and Cantor as representatives of the underwriters for the offering. The aggregate net proceeds to the Company from the offering were approximately $37.7 million, after deducting the underwriting discounts and commissions and offering expenses, for the year ended December 31, 2018. There were no proceeds from this offering for the quarter ended March 31, 2019.

In March 2019, the Company entered into a Controlled Equity Offering Sales Agreement with Cowen and Company (“Cowen”), as sales agent, pursuant to which the Company has the right to offer and sell, from time to time, through Cowen, shares of the Company’s common stock, par value $0.001 per share, having an aggregate offering price of up to $40.0 million, by any method deemed to be an “at the market offering” as defined in Rule 415(a)(4) under the Securities Act of 1933, as amended (the “Cowen ATM Offering”). The shares will be offered and sold pursuant to the Company’s shelf registration statement on Form S-3 (File No. 333-229045.

As of March 31, 2019, there have been no shares sold under the Cowen ATM Offering and costs in connection with the S-3 offering of approximately $0.2 million have been capitalized as deferred offering costs in the accompanying condensed consolidated balance sheets as of March 31, 2019.

A summary of the Company’s stock option activity for the three months ended March 31, 2019 is as follows:

	Number of Shares			Weighted- Average Exercise Price Per Share		Weighted- Average Remaining Contractual Life (Years)		Aggregate Intrinsic Value
Balance at December 31, 2018		2,047,237		$	3.07		7.6	$	366,007
Granted		—			—
Exercised		(21,871	)		2.36			$	8,306
Forfeited		(36,797	)		3.15
Expired/Cancelled		(10,782	)		4.81
Balance at March 31, 2019		1,977,787		$	3.07		7.5	$	320,766
Exercisable at March 31, 2019		1,288,938		$	2.93		6.6	$	307,101

As of March 31, 2019, there was total unrecognized compensation expense of $1,427,673 related to unvested stock options, which the Company expects to recognize over a weighted-average period of approximately 2.89 years.

A summary of restricted stock unit (“RSU”) activity for the three months ended March 31, 2019 is as follows:

	Number of Shares			Weighted- Average Grant Date Fair Value Per Share
Balance at December 31, 2018		227,707		$	3.39
Granted		15,000			2.31
Released		(13,126	)		3.40
Balance at March 31, 2019		229,581		$	3.32
Vested and unissued at March 31, 2019		15,001		$	3.26

Vested and unissued awards at March 31, 2019 represents RSU awards granted on August 16, 2018 for which the vesting date was March 31, 2019, but for which issuance of the awards occurred on the next business day, April 1, 2019. Unrecognized compensation expense related to the remaining unvested RSUs was $668,219 at March 31, 2019, which the Company expects to recognize over a weighted-average remaining service period of 3.05 years.

Stock-based compensation expense recorded in the accompanying condensed consolidated statements of operations for the three months ended March 31, 2019 and 2018 was as follows:

	Three Months Ended March 31,
	2019		2018
Selling, general and administrative	$	181,981	$	1,101,992
Research and development		48,450		30,909
Cost of product and product-related services revenue		10,815		10,557
	$	241,246	$	1,143,458

Stock-based compensation expense for the three months ended March 31, 2018 included $1.0 million of selling, general and administrative compensation expense relating to the issuance of 259,551 shares under RSUs granted to the Company’s executive officers in January 2018 at a grant date fair value of $3.84 per share. The RSUs vested in full on January 29, 2018.