6. Fair Value Measurements

Certain of the Company’s financial instruments are not measured at fair value on a recurring basis but are recorded at amounts that approximate their fair value due to their liquid or short-term nature, such as accounts payable, accrued expenses and other current liabilities.

Fair Value of Caelum

The Company values its investment in Caelum in accordance with ASC Topic 820, Fair Value Measurements and Disclosures, and as of December 31, 2020, estimated the fair value to be $17.6 million based on a per share value of $2.43. As of December 31, 2020, the following inputs were utilized to derive the value: risk free rate of return of 0.36%, volatility of 70% and a discount for lack of marketability of 21.0% to 31.0% based on maturity dates of various scenerios.  Further, the Company considered the impact of the acquisition of Alexion by AZ, which if consummated, will shorten the timeframe in which the option will be exercised in accordance with the A&R DOSPA.

As of December 31, 2019, the estimated fair value of the Company’s investment in Caelum was $11.1 million based on a per share value of $1.54. As of December 31, 2019, the following inputs were utilized to derive the value: risk free rate of return of 1.6%, volatility of 70% and a discount for lack of marketability of 28.7%.

Caelum Warrant Liability

The fair value of Caelum's warrant liability, which was issued in connection with Caelum’s convertible note, was written up to the full value of the liability prior to the conversion of the notes in January 2019 (see Note 10). The fair value was measured using a Monte Carlo simulation valuation methodology. A summary of the weighted average (in aggregate) significant unobservable inputs (Level 3 inputs) used in measuring Caelum’s warrant liabilities that are categorized within Level 3 of the fair value hierarchy as of January 2019 was as follows:

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In connection with the DOSPA Caelum's convertible notes automatically converted into common shares of Caelum and the warrant liability payable to the placement agent in connection with the placement of the convertible notes was also issued (see Note 10).

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Caelum Convertible Notes

Caelum’s convertible debt was measured at fair value using the Monte Carlo simulation valuation methodology. A summary of the weighted average (in aggregate) significant unobservable inputs (Level 3 inputs) used in measuring Caelum’s convertible debt that is categorized within Level 3. As of December 31, 2018, conversion of the Caelum Convertible Notes was probable and as such the fair value approximated cost. The Caelum Convertible Notes were converted during 2019. As of January 2019 the following inputs were utilized to derive the notes’ fair value:

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Cyprium Warrant Liability

The fair value of the Cyprium Contingently Issuable Warrants in connection with the 2018 Venture Debt was determined by applying management’s estimate of the probability of issuance of the Contingently Issuable Warrants together with an option-pricing model, with the following key assumptions:

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The following tables classify into the fair value hierarchy of Fortress’ financial instruments, measured at fair value on a recurring basis on the Consolidated Balance Sheets as of December 31, 2020 and 2019:

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The table below provides a roll forward of the changes in fair value of Level 3 financial instruments for the years ended December 31, 2020 and 2019:

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