Note 6 - Stock-based Compensation

6 Months Ended

Jun. 30, 2018

Notes to Financial Statements

Disclosure of Compensation Related Costs, Share-based Payments [Text Block]

6. Stock-based Compensation

Stock Options

The following table summarizes stock option activity for employees:

	Options			Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Outstanding at December 31, 2017		2,681,072		$	7.18		6.8	$	121
Granted		1,891,600		$	2.65
Exercised		-		$	-
Forfeited		(83,433	)	$	5.82
Expired		(42,013	)	$	13.30
Outstanding at June 30, 2018		4,447,226		$	5.22		7.9	$	614
Exercisable at June 30, 2018		1,844,871		$	7.44		6.1	$	286
Vested or expected to vest at June 30, 2018 (1)		4,447,226		$	5.22		7.9	$	614

_____________________

(

1

) Represents the number of vested options at

June 30, 2018

plus the number of unvested options expected to vest based on the unvested options outstanding at

June 30, 2018.

Employee Stock Purchase Plan

The

2014

Employee Stock Purchase Plan (ESPP) initially authorized the issuance of up to

140,500

shares of Common Stock. The number of shares increases each

January 1,

commencing on

January 1, 2015

and ending on (and including)

January 1, 2024,

by an amount equal to the lesser of

one

percent of the outstanding shares as of the end of the immediately preceding fiscal year,

281,000

shares and any lower amount determined by the Company’s Board of Directors prior to each such

January

1st.

The Company’s Board of Directors determined there was to be

no

increase on

January 1, 2018.

As of

June 30, 2018,

the

2014

ESPP authorized the issuance of up to

304,991

shares of Common Stock. The

seventh

offering under the

2014

ESPP began on

January 1, 2018

and ended on

June 30, 2018.

51,984

shares were issued during the

three

and

six

months ended

June 30, 2018

under the

2014

ESPP. The Company incurred

$24,000

and

$48,000

in stock-based compensation expense related to the

2014

ESPP for the

three

and

six

months ended

June 30, 2018,

respectively. The Company incurred

$52,000

and

$0.1

million in stock-based compensation expense related to the

2014

ESPP for the

three

and

six

months ended

June 30, 2017,

respectively.

Common Stock

The Company has the following shares of Common Stock reserved for future issuance:

	June 30,		December 31,
	2018		2017
Conversion of Series A Preferred Stock		22,112,775		22,112,775
Stock-based compensation awards		5,163,957		3,572,457
Employee Stock Purchase Plan		140,479		192,463
Total		27,417,211		25,877,695