4. FAIR VALUE MEASUREMENTS

Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs.

The three-level hierarchy for the inputs to valuation techniques is briefly summarized as follows:

Level 1 – Valuations are based on quoted prices in active markets for identical assets or liabilities and readily accessible by the Company at the reporting date. Examples of assets and liabilities utilizing Level 1 inputs are certain money market funds, U.S. treasuries and trading securities with quoted prices on active markets.

Level 2 – Valuations based on inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Examples of assets and liabilities utilizing Level 2 inputs are corporate bonds, commercial paper, certificates of deposit and over-the-counter derivatives.

Level 3 – Valuations based on unobservable inputs in which there is little or no market data, which require the Company to develop its own assumptions.

The following table sets forth the fair value of the Company’s financial assets and liabilities measured on a recurring basis by level within the fair value hierarchy (in thousands):


	December 31, 2019
	Total Fair Value		Level 1		Level 2		Level 3
Assets:
Money market funds	$	147,208	$	147,208	$	—	$	—
Corporate bonds		33,134		—		33,134		—
Commercial paper		56,041		—		56,041		—
Asset-backed securities		8,005		—		8,005		—
Total	$	244,388	$	147,208	$	97,180	$	—

Liabilities:
Derivative liability for Exit Fee	$	969	$	—	$	—	$	969
Total	$	969	$	—	$	—	$	969


	December 31, 2018
	Total Fair Value		Level 1		Level 2		Level 3
Assets:
Money market funds	$	73,238	$	73,238	$	—	$	—
U.S. treasury securities		3,996		3,996		—		—
Corporate bonds		34,590		—		34,590		—
Commercial paper		43,154		—		43,154		—
Asset-backed securities		9,378		—		9,378		—
Total	$	164,356	$	77,234	$	87,122	$	—

Liabilities:
Derivative liability for exit fee	$	533	$	—	$	—	$	533
Foreign currency derivative contracts		52		—		52		—
Total	$	585	$	—	$	52	$	533

Where quoted prices are available in an active market, securities are classified as Level 1. The Company classifies money market funds, U.S. treasury securities and U.S. treasury notes as Level 1. When quoted market prices are not available for the specific security, the Company estimates fair value by using benchmark yields, reported trades, broker/dealer quotes and issuer spreads. The Company classifies corporate bonds, commercial paper, asset-backed securities and foreign currency derivative contracts as Level 2. In certain cases, where there is limited activity or less transparency around inputs to valuation, securities are classified as Level 3. There were no transfers between Level 1 and Level 2 during the periods presented.

In May 2018, in connection with entering into the Loan Agreement, as defined and discussed in Note 6, the Company entered into an agreement pursuant to which the Company agreed to pay $1.5 million in cash, or the Exit Fee, upon any change of control transaction in respect of the Company or if the Company obtains both (i) FDA approval of tenapanor for the treatment of hyperphosphatemia in CKD patients on dialysis and (ii) FDA approval of tenapanor for the treatment of patients with irritable bowel syndrome with constipation, or IBS-C, which was obtained on September 12, 2019 when the FDA approved IBSRELA® (tenapanor), a 50 mg, twice daily oral pill for the treatment of IBS-C, in adults (the “Exit Fee Agreement”). Notwithstanding the prepayment or termination of the Term Loan, the Company’s obligation to pay the Exit Fee will expire on May 16, 2028. The Company concluded that the Exit Fee is a freestanding derivative which should be accounted for at fair value on a recurring basis. The estimated fair value of the Exit Fee is recorded as a derivative liability and included in accrued expense and other current liabilities on the accompanying balance sheets.

The fair value of the derivative liability was determined using a discounted cash flow analysis and is classified as a Level 3 measurement within the fair value hierarchy since the Company’s valuation utilized significant unobservable inputs. Specifically, the key assumptions included in the calculation of the estimated fair value of the derivative instrument include: i) the Company’s estimates of both the probability and timing of a potential $1.5 million payment to Solar Capital Ltd. and Western Alliance Bank as a result of the FDA approvals, and ii) a discount rate which was derived from the Company's estimated cost of debt, adjusted with current LIBOR. Generally, increases or decreases in the probability of occurrence would result in a directionally similar impact in the fair value measurement of the derivative instrument and it is estimated that a 10% increase (decrease) in the probability of occurrence would result in a fair value fluctuation of approximately $0.1 million.

Changes in the fair value of recurring measurements included in Level 3 of the fair value hierarchy are presented as other income (expense), net in the Company's statements of operations and were as follows for the year ended December 31, 2019 (in thousands):


	Estimated Fair Value
	of Derivative Liability
Balance of Level 3 Liabilities at December 31, 2017	$	—
Initial estimated fair value of derivative liability for Exit Fee in May 2018		546
Change in estimated fair value of derivative liability for Exit Fee		(13)
Balance of Level 3 Liabilities at December 31, 2018		533
Change in estimated fair value of derivative liability for Exit Fee		436
Balance of Level 3 Liabilities at December 31, 2019	$	969

The carrying amounts reflected in the balance sheets for cash equivalents, short-term investments, accounts receivable, prepaid expenses and other current assets, accounts payable and accrued expenses approximate their fair values at both December 31, 2019 and December 31, 2018, due to their short-term nature.