result of a decrease in CRO costs and related research consulting of $11,000 for the ramp up of the PK Study in 2019 and a decrease of $74,000 in drug development costs. All of the research and development expenses for the six months ended June 30, 2020 and 2019 were external.
General and Administrative Expenses
General and administrative expenses increased by $120,958, or 17.0%, to $833,368 for the six months ended June 30, 2020 from $712,410 for the six months ended June 30, 2019. The increase was primarily the result of an increase in salaries and benefits of $269,000, accounting and auditing fees of $67,000, and legal fees of $70,000 offset by a decrease in guaranteed payments of $88,000 and a decrease in marketing and business development costs of $189,000.
Liquidity and Capital Resources
Since our inception in 2012, we have devoted most of our cash resources to research and development and general and administrative activities. We have financed our operations primarily with the proceeds from the sale of membership interests and convertible promissory notes. To date, we have not generated any revenues from the sale of products and we do not anticipate generating any revenues from the sales of products for the foreseeable future. We have incurred losses and generated negative cash flows from operations since inception. As of June 30, 2020, our principal source of liquidity was our cash, which totaled $1,048,492.
Financings
For the year ended December 31, 2019, we received net proceeds of $2,371,508 from the sale of convertible promissory notes. For the year ended December 31, 2018, we received net proceeds of $1,150,468 from the sale of redeemable preferred convertible interests. For the six months ended June 30, 2020 and 2019, we received net proceeds of $1,824,052 and $1,807,903, respectively, from the sale of convertible promissory notes.
Debt
In 2019, we issued an aggregate of $3,675,000 principal amount of convertible promissory notes, $1,245,000 of which were related to the conversion of redeemable preferred convertible interests that were issued in 2018. All of the notes were outstanding for the year ended December 31, 2019. For the six months ended, June 30, 2020, we issued an aggregate of $1,869,133 principal amount of convertible promissory notes. These notes will convert to common interests that results in the greater ownership percentage of the price issued in connection with this offering or at the price of $400,000 per 1 % of common interests. Regardless of the price per membership interest used in this offering, the notes shall convert into common interests at a rate no less than 1% per $400,000 of preferred interests, placing a cap on the valuation of the Company at $40,000,000 for the purposes of the conversion, even if the IPO offering is higher. There was no debt outstanding for the year ended December 31, 2018.
Non-voting Preferred Members’ Interest
In 2018, we sold an aggregate of $1,320,000 of redeemable preferred members’ interests. In 2019, $1,245,000 of the redeemable preferred members’ interests were converted into convertible promissory notes. The principle of the preferred members’ interests will be repaid upon the closing of this offering. For the years ended December 31, 2019 and 2018, there were principle amounts outstanding of $75,000 and $1,320,000, respectively. For the six months ended, June 30, 2020 and 2019, there were principle amounts outstanding of $75,000 and $780,000, respectively.
Future Capital Requirements
We expect that the net proceeds from this offering and our existing cash will be sufficient to fund our operations and capital requirements for at least the next 18 months. We believe that these available funds will be sufficient to complete our Phase 2b clinical trial for IMC-1 and commence the planning of our Phase 3 study in FM for this product candidate. However, it is difficult to predict our spending for our product